We seem to have had lots of these threads lately. Here's what's what in DF computers.
GPU is irrelevant; it's generally good to have something better than integrated, but you shouldn't worry or care about it.
4gb ram is not necessarily optimal. Use the LAW .exe and go with 6 or 8 gb of ram. Remember, your computer will be doing stuff in the background. Because of the astonishing number of memory operations DF makes, you want the lowest CAS latency possible for the best performance.
You do NOT want to use a dual-core CPU, and you do NOT want to base your buy on how many ghz it has. Base your purchase on benchmarks; clock speed is utterly useless as a measurement. Unless you really want to tell me that my i5 2500k is really just about the same as 4 of my 8-year-old Dell's Pentium 4s put together. In which case I'm going to slap you.
Hyperthreading is irrelevant and useless for DF. The i5 2500k is a staple for modern gaming rigs and is excellent with DF, not to mention only $200. If you don't know a lot about tech, just get the 2500k. Overclock it for max performance if you have the cooling and knowhow. Don't be surprised if you don't see one of your cores maxed out, Windows is amazing with shifting it from one core to the next.
An SSD will probably do beautiful things to the amount of time it takes to save/load games; mine is in the mail right now. Not vital for good DF, but nice to have.
To avoid having tons of threads like this, can we get a sticky with information compiled about building a DF-centric computer, and general info on what you want and what you don't?