new used computer advice needed

Hello, I mostly play games, I have an off the shelf Gateway with an
Athlon 64 3400+, 1GB Ram, and a BFG 6800OC 128MB AGP Video Card. This
setup games pretty good, I can play HL2 and Doom 3 at decent
framerates. I bought a G4 Mac Tower for my son, who was into video
editing, but he has decided he would now rather have a PC. No big deal,
I bought a used one, it is a P4 2.6C 800Mhz FSB with HT, on a Abit IS7
MB, with a SATA 160GB drive and 1GB Ram. It would seem the latter
machine is quite a good setup for overclocking, with an identical video
card, which machine would be better for gaming??? the P4 overclocked,
or the Athlon64 stock?

Jerry