would say minimum to play all games on the market in atlest medium, would probably be, 400$ w/o OS.
Can get a low end 775 cpu and overclock it to 3.5ghz+
Or get a AM3 dual core.
Minium gfx card i would get to have a decent gaming rig would be a 4830HD or 9800GT.
I have a 4670 and it does fine on COD4 n such but when it comes to the more demanding games, it struggles.