Go ahead and buy a nice power supply now, as well as the 7600gs.
The power supply you can then reuse in your next build, and you can OC the 7600gs and it will be able to handle COD much better than what you currently have.
About fps: Any amount of FPS over your refresh rate on your monitor is wasted frames. Most LCDs have a refresh rate of 60hz, while nice CRTs can have up to about 80hz. Your monitor can only show so many fps, so if your graphics card is spitting out more than that your monitor isn't showing them. I like to keep my FPS somewhere in the 50-60-70 range, anything above that turn the quality up, anything below that turn the quality down.
AMD Athlon 64 X2 4200+ @ 2.6
ASUS A8N-SLI Premium
2x GIGABYTE 7600GT Silent Pipe SLI @ 607/800
OCZ Platinum 2GB DDR 400 @ 220 2.5-3-3-5
Raptor 74GB 10,000 RPM