Athlon X2 2.1GHz + 2GB ram + HD3850 here, No reason to upgrade.
No real problems with games like Fallout NV/Crysis/DiRT2. I jack up the settings all the way and put them down a little and they run fine.
Today I wouldn't go with anything less than that - dual core, 2GB ram, and a high end video card from a few generations back.
Computers have become fast enough that we no longer are utilizing their full potential.
