Form factor is what I refer to when I'm talking about micro ATX. It's the size of the motherboard. You have ATX, which is standard, micro which is about half as large, mini I think which is even smaller, and a bigger form factor than standard ATX as well. Micro-ATX builds cram a lot of hardware into a small space, and it can limit the range of graphics card you get due to size. Some hardware also either does or doesn't come in Micro-ATX form factor, further limiting your selection.. Micro-ATX is appealing to some who like a smaller desktop computer because you end up with a little box for a case that's more portable, not a big honkin' tower. But they're known to have heat issues because the hardware is packed in there so tightly.
A very rough guide to quality/performance:
Motherboards:
It's what all the parts of the computer plug into, and basically determines what performance level of parts you can run with and in what amounts you can use. What motherboard, and who makes it, also determines what kind of software it has, which may or may not have features that are helpful or good, like support for 5.1 sound. You don't need a great mobo, but you do need a not-shitty one because if it has problems later on, your system is kinda screwed.
Barebones low-end mobo. $20 to $30. Supports low end processors which are probably out-dated, supports less total system memory, might have an older PCI Express slot (what your GPU goes into) that doesn't run as fast, will have fewer USB ports and other sundry ports, may have pretty low quality motherboard software, may have slightly lower performing ethernet connection.
Average mobo. $35 to $60. Supports a fairly wide range of processors to from low-end to high-end ones, supports a modest (probably 16 or so gigs) of memory, probably has quite a few USB and other ports, a PCI Express x16 slot, decent software with your standard 5.1 or 7.1 sound support, standard ethernet speed connection
Performance mobo. $70 to
Motherboards for enthusiasts. Roughly speaking it supports newer and higher-end processor socket types, supports a lot of memory (32 gigs+), has multiple PCI-Express slots, tons of USB and other doodad connectors, lots of extra features on the motherboard, lots of performance tweaking and utility suites of programs. Generally overkill once you're spending more than $100 on it.
Processors (CPU) (I only do Intel)
i3 - Light weight duo or multicore processors. Decent for low end games and general computing.
i5 - Mid-grade multicore processors. Pretty decent for the price, a good compromise between performance and cost.
i7 - High-grade multicore processors. For people with money to burn on performance, enthusiasts and people who need a lot of processing power.
Within each group of processors there's different models and clock speeds so the price within each one can vary a bit, especially the i7s. If you really want a good DF computer, you'll get a decent processor.
System Memory (RAM):
2gigs of memory - Enough for casual and older games but not a lot else. Lots of modern games want at least 2 gigs to themselves, and you have to account for Windows using up some of that 2 gigs already.
4gigs of memory - Adequate for most games, although you will likely have to compromise on some setting for games that ask for a minimum 4 gigs of available memory.
8 gigs of memory - What I consider an average amount of memory for a decent gaming rig. Having more than games want generally improves performance and load times because the game and windows aren't fighting over system resources. Almost no games exceed this as a recommended spec.
16 gigs of memory - Generally slight overkill, but having 16 gigs myself, I'm liking having that much extra memory for gaming.
32gigs+ of memory - For enthusiasts and business servers.
You'll want a lot of available memory for DF, since it uses increasing amounts the longer worlds go on.
Video card (GPU)
Integrated Graphics - With Intel, the CPU can run double duty as the CPU and GPU, meaning you don't
have to have a video card. Lots of motherboards/CPUs from Intel ship with this capability, even though it's only really relevant for laptops and non-gaming computing. If you honestly care about gaming, you do not want to run on your integrated graphics. They also vary wildly in terms of what they're capable of doing besides raw horsepower, so you're often rolling the dice buying new games which may or may not work well/at all with them, for reasons.
512 mb of memory card - Basically only good for casual games or for a lot of games made before 2000. It's pretty much inadequate for modern gaming. In some ways integrated GPUs have already come up to this level of performance in PCs.
1gig of memory card - A decent amount for mid-spec settings on modern games.
2 gigs of memory card - An abundant amount, letting you push settings higher (assuming you have decent memory and CPU as well)
4 gigs+ of memory - A lot of memory on a video card. For enthusiasts and graphics and design professionals pretty much.
GPU isnt anywhere near as important to DF as CPU and memory.
Power Supply (PSU)
On the surface you care about how many watts of power it runs. There's a few websites out there that help you figure out how big of a PSU to get. But generally if you go the middle of everything I've listed here, a 550watt PSU should suffice. I would not skimp on your PSU by trying to find a cheap one. There's other stuff to care about, but generally, try and find one's that are at least 80+ Gold Standard or higher. Not because the power efficiency is so important, but because to reach that kind of efficiency, it has to have quality materials, so 80+ Gold PSU tend to be more reliable products. Your PSU dying is not something you want to have happen.
Hard Drive (HDD)
You have two real choices. Serial ATA (SATA) or Solid State Drive (SSD).
SATA are standard hard drives these days. They're about as fast as they can get in your standard home PC.
SSD are different and special and basically take no time to look up your data, which means loading screens are almost a thing of the past with them. They have a few downsides. 1) They're more expensive than standard hard drives. 2) They generally come with a smaller storage capacity for what you pay.
In terms of storage:
100 gigs of HDD space is enough for an operating system, a lot of older games or relatively few newer games.
200 - 500 gigs of HDD space is enough for pretty much unlimited amounts of older, smaller games and a good number of installed new games.
1 Terabyte (1000 gigs) is enough for all your media collection and a lot of newer-sized games.
2+ Terabytes is enough you actively have to be trying to fill the drive up. Only servers, businesses, game pack rats and habitual torrenters make full use of that kind or bigger storage.
Most new big budget AAA games want anywhere between 5 and 40 gigs of hard drive space. GTA V, for example, wants 65 gigs of hard drive space for their recommended spec.
USB ports
These are the ports you connect most peripheral devices to a computer with. Game controllers, cell phone chargers, some headphones and speakers, mouse and keyboard....
There's USB 2.0 and USB 3.0 right now. USB 3.0 is newer, is faster and more responsive, and some newer devices make special use of USB 3.0 to do special things. USB 3.0 isn't critical unless you're gadget happy IMO, but it is generally better than 2.0. The more USB ports a motherboard offers, the better.
Computer Case:
This kinda ties back to form factor, but basically you have:
micro ATX cases, small box-like cases that are easy to move around and don't take up much space.
medium towers, your garden variety-sized desktop PC. Weighs about 10 or so pounds fully loaded.
full towers, larger than average cases. These cases tend to be aimed at gamers. Can weigh between 15 and 25 pounds fully loaded.
server towers, obviously for people who run servers or just like an excessively big case. At least 20 pounds of giant awkwardness.
There might also be some goofy special form factors out there like slim or what have you. A regular old ATX motherboard will fit into a medium tower pretty snugly with little extra room. You can put a regular ATX motherboard into a larger size case if you want extra room and space to upgrade. Even if you're not going to ever put your hands inside it yourself, you should be mindful that how much space is in the case, what the clearances are and all that, relates back to how big or tall of a graphics card you can fit. (And technically your CPU cooler, but I doubt you'd fool with that.)
This is the website I built my last 3 PCs off of:
http://www.logicalincrements.com/Even if you're having a shop put it together, it would behoove you to research any parts they recommend you. The people you're dealing with might be gamers and super cool, or they might be dicks who buy old hardware cheap and sell it to people who come into their shop marked up to today's prices. (I generally have a pretty low opinion of computer shops because they can easily exploit people's ignorance.) I use Newegg to buy my parts and price compare to Amazon usually.
All that above doesn't even really get down to finding the best performing hardware within each category and performance range, as there's lots of things about a piece of hardware that affect performance. (like the clock speed on the graphics card, for one.) But they're honestly not that important for you to know about. Just going by the raw numbers listed here you can get roughly in the range of performance and quality I laid out. A really important thing to do when looking at parts is look at your motherboard specs. It says what it supports, and between the range of numbers and the prices when you look up those parts, you can get a rough estimate of quality/performance.