Since you have a P4 computer I dont know how well it will play out with a 8800gt, the 8600gt will play WoW with no worry and probably COD 4 at a decent setting maybe not max out though (because I play COD 4 on my m8600gt on my laptop and almost max the settings out) . Hope it helps.
the 8600GT is much better, but make it sure though that you are getting the DDR3 version,....and if you can find one, a factory-overclocked version will give you a bit of a performance boost than 8600GT's sold at stock clock speeds.