I'm not saying that your card can run any game. I'm just saying that OC'ing it won't result in too much more performance. I OC all my cards however. You're probably better off tuning the settings in CoD4 to allow higher fps. Its been awhile since I've played the CoD series so I don't remember most of the settings that help with fps. You should check out some cod4 or gaming forums to see what settings you can change.
I realized I didn't really answer your question before. So here's a better response. For flashing, you can look here: http://forums.laptopvideo2go.com/top...bile-gpu-roms/
. Bear in mind these flashes are to unlock the card for software OC'ing. This is a bit different than hardwired OC'ing directly in the rom (which is what you were doing with the 7900 GS). Anyway, you may be able to use the old disc you have to flash your 7900 GTX. It all depends on if your 7900 GTX is rev0 or rev1 though. It sounds like you're software OC'ing already so all of this info is probably irrelevant.
Your core can probably be OC'ed anywhere from 575 to 600. Your memory can probably be OC'ed anywhere from 750 to 850.