Why buy an old tech processor?
Core 2 Duo is the way to go.
Very cheap called Pentium Dual core
E2140, E2160, E2180, E2200
Low budget C2D:
E4500, E4600, E4700
E6550, E6750, E8200,
E8400, E8500, Q6600, Q9300, Q9450
We need info on your hardware so we can help you choose.
Motherboard maker + model
RAM you said 1GB DDR2 (single or dual channel)?
LCD/CRT + resolution
Geforce 8600GT and 8800GT are worlds apart.
Upto 1280x1024 screen resolution and low details you could go with 8600GT. Anything above that and 8800GT is the way to go.