Shot in the dark, but try the Beta 260.xx drivers. Seems to improve my system, maybe not in FPS as much, just in stability. But due to you having a single Card, maybe it will imrpove more towards the performance side of things.
The GTX480 is pretty much the top GPU to have, not much will beat it. If you could, try your friends PSU in your machine, or try another PSU in your machine.
What would be a good thing to do aswell with your PSU is to take all the optical drives and just have your system HDD (One with Windows on) and do the benchmark test. This give a little more free power for the GPU.
But givin that i dont think its a lack of power as such, more of a case of "bad" power, hense why it plays, just not flawlessly.