This is the PC that I'm currently building. I decided to go with the dual core 3.1 since this will be a gaming PC only. I'll upgrade to a Quad with DDR3 memory in the next 2~3 years if and only if I'll notice a difference. For now, I'll be able to play all games at high settings and I don't think I'll have a problem with StarCraft 2 or Diablo 3. I strongly believe that Crysis is a poorly designed game that consumes way too many resources inefficiently so please don't use this game as a Benchmark.
What do you think for a budget system?
I'm sort of tempted to get the XFX Radeon HD 5870 850MHZ 1GB GDDR5 4.8GHZ however I don't know if I'll notice a difference.
I dont understand why you dont just step up to an AM3 motherboard and DDR3 RAM. There is not enough price difference to justify building a new system that is a generation behind. If you have an AM3 motherboard you will eb able to drop in an AMD quad or even the next gen 6 core AMD cpu, with an AM2+ you may be bios locked into older only CPU models.