A PSU alone will not cause you bad frame rates, thats all the CPU, GPU, and RAM. If the PSU wasn't up to it, you'd know because the computer would just shut off(I know cause my old PSU did that with my X800).
The best thing you could do is find a good CPU for 939 if you want a temporary fix. The best thing, but slightly more expensive route, would be to find a cheap AM2 and processor. I went from a socket 754 to my AM2 this past July, I went with a highend mobo but I got a computer that can play any new game just fine(COD4 maxed without AA) and I have the option of crossfire too. The upgrade was around $550, but CPUs have fallen about $50, the ram you can get for $30 now, I paid $100 and you already have a great video card.
PS3 Games: GTA:IV & 5, GT5!, Killzone 2, MGS4, MAG
360 Games: GTA:IV, Crackdown 1&2, Fora 2/3/4
PC Games: CSS, GTA5, PayDay 2, Planetside 2, Minecraft, FarCry