Well, I did it (GeForce 9800GTX). And
wow is all I can say...

Unfortunately I am not a genius with hardware related stuff, and didn't know anything about power supply, lol. So it's up and running, and I noticed great improvements esp in COD4 (wow!), and SHIV looks awesome now with nice framerates (hate that post process filter thing, but environmental, volumetric, etc very cool!) -- but... looks like I'll need a new powersupply because some nVidia app is telling me it's not getting enough juice, and it has tuned the performance down a bit to prevent damage (kinda neat that it'd do that). Not sure how much further improvement I'll see when I have enough wattage in the PC and the restraint comes off, but as it is even right now, I'm impressed.