Even with your old P4 you should see a pretty good upgrade from a crappy 8400GS. If you have a single core system though, it will probably only be as good as a $55 5570 for you. You will of course get more out of it if you have a dual core system, or at least Hyper Threading. Overclocking your CPU will also help (turn your PC into a winter heater
). Exactly which P4 do you have?