Both are 8 pixel pipelines. The 6600 GT is only a 128 bit memory interface vs 256 bit on the 6800 XT, but the 6600 GT GPU clock is 500 vs 300-350 mhz on the 6800 XT. Both are 1 ghz memory clock.
If I was Nvidia I would be ashamed to call it a 6800. Looks like marketing hype intended to sucker in people who think they will be getting a $250 card for $130.
You need to look at specifi cards with the 6800 XT. The GPU clock varies a lot. One benchmark I found showed a GPU clock speed of 450 Mhz on the card they tested vs 300-350 for all the cards listed on newegg. Memory speed varies from brand to brand also. Definitely a low budget card.