So, for some actual numbers, 3 GHz is pretty good for a single processor. These days it's a good idea to have at least a dual-core processor, but probably not for the games you're looking to play. Still, see if you can find an affordable machine with something like a 2.5 GHz dual-core. It might make your machine a bit more future-ready.
With RAM, you won't be able to use more than 4 GB with a 32-bit processor even if you have it, but the sky may as well be the limit with a 64-bit processor. 4 is still pretty good though. 2 should be fine for your goals.
Video cards have a really high turnover rate in the market, so rather than look at the specs, I choose based on price. I don't know the difference between nVidia and ATI cards (those are the two major manufacturers), but I can tell you from personal experience that nVidia makes the most horrible and feature-deprived user interfaces known to man. DF has a pretty bad UI as an alpha, but nVidia's control panels are worse and more buggy as finished products. I don't know if ATI is better but I'm going to find out soon because I'm switching. Anyway, I look for the card that costs $200 to $300 and gets good reviews because that's what the Good Card O-The Week will be. There's also ones that are $400 or $500, but I don't need that much. For you, I would recommend spending about $60 to $100 on a card, but when you find one, try to look up what reviews it's getting so you can get an idea of how good it is and how hot it tends to run. If it sounds like the video card is taking up a large percentage of your budget, don't worry, because it should. CPU and video card together should be around half the price of the entire machine. If you're getting a premade machine, find out what the video card is (if it doesn't have one, find another premade PC that does) and look online to find out what that card costs by itself.
We all have our own ideas of how much storage space is enough. 1 TB (1,000 GB, or 1,000,000 MB) is popular right now. I prefer having a relatively small primary drive where the OS and programs live, and a large secondary drive to store all my files on. If you install lots of games, download lots of media, and are not in the habit of deleting things, then more is better. 500 GB is still pretty good if you're not a huge media user though, so you could save a bit of money for other things by getting a system with a smaller hard drive, if you can find one. A PC-builder will fix you up with whatever combo you want.
So try finding a system with those specs, and if that doesn't work (and even if it does), you should try to find a small PC-builder. They'll let you fully customize it for a better price, and they'll give more helpful advice too. I'm serious about this. It's a bit more work and you don't get it as quickly, but you'll be happier with your machine for the duration of its working life. No, I'm not some sort of marketing plant for them, though I know I sound like it. I just have seen what's available at the big-box stores and what's available through the little guys, and the big box stores got nuthin' on the little guys.