Hi
have a look at www.xfxforce.com that will give you information on the 6600 and the 6800, I have the 512mb pci expressx16 nividia 6600 and its ok for the games I play, but the 6800 looks even better with faster clock speeds.
Neither. Join the future and get nothing less than a 7600GT/X1800GTO. If you're looking to overclock, I'd recommend the ATI; it'll overclock to near-XT speeds and unlock to 16 pipes with a little luck. All of that for $200. And if you get a Crossfire motherboard, you can buy another one when you have the $$$