i went out and bought a 520W PSU for $75, and installed that and the radeon hd 4850. i installed the drivers, but now windows xp is taking AGES to boot up (it gets past the logo, shows the mouse on a black screen). when rebooting for the driver installation i got a BSOD, something about pci.. i forgot ><
and now CoD4 is really really laggy... 21-50 FPS and i said 'optimal system settings' :S
maybe it's overheating? it runs really hot...
Overheating, while not entirely impossible, would likely cause the computer to shut off or restart completely rather then slowing it down. Windows does tend to get "confused" if you install new hardware, I've had scenarios where I would install a stick of RAM (tested good) and the computer would fail to boot yielding a BSOD on every boot till I took the stick out. I re-formated that machine and put xp back on it, and with the stick in after a fresh install it ran fine.
But before you just wipe everything (unless this is something your ok with) you should try a defrag and a virus scan.
A few of things to try:
-make sure that the old video card drivers are uninstalled, that windows isn't trying to use both at the same time
-disable any bios settings for shared video memory
-if the previous video card was integrated in the motherboard, disable it with the BIOS
-check BIOS settings for the card, especially if the new one has a different interface (PCIx, rather than AGP, for example) than the old one.
-if you have a spare hard drive, switch it out and put a fresh install on the spare, and see if that is slow the same way. If the fresh install on the spare drive is also slow, don't waste your time re-installing windows.
-make sure you have correct/updated drivers for the card
-make sure the problem is the new card by switching back to the old one temporarily, and seeing if that fixes the problem.