It would probably just play games on low-medium settings. For example, I got AMD Athlon 64 2.0ghz
7600GT 512mb GDDR2
1gb DDR RAM
And I can play most games. But some like UT3 and Cod5 just work.
You want something like a quad or a dual core. Preferably over 2.4ghz. Atleast 2gb DDR2 RAM and for NVIDIA a 8800, 8600, 9600, 9800 or GTX series will do.
For ATI you want the HD48xx series. Atleast 512mb.