I think the real question here is, "What kind of games are you going to be playing?"
A lot of the people who have posted in this thread have tossed around a lot of data, model numbers and comparisons. That's cool to be hardcore (gamers... we are a fanatical bunch!) but it's all relative to what games you're playing, or expect to be playing over the next few years.
Since I'd guess a hardcore gamer would (a) know exactly what they want to buy, and (b) not particular care about a budget, just so as long as they got what they wanted, I'm guessing you're a casual gamer who wants a quality rig at a good price.
If you're just wanting to play some games like World of Warcraft or Quake 4 and don't want to break the bank, I'd recommend going with a solo core, and invest the savings in plenty of RAM and a great video card, which is really what counts when you're playing games.
For RAM, get a minimum of 1GB of quality memory, preferrably with heatsinks. Get more if you can afford it--there's no such thing as "too much RAM".
For a video card, buy the newest thing you can afford and you'll be set. Just make sure it's compatible with your motherboard, and that you have an adequate power supply to push it.
Best of luck!