I've been looking at this too.
6600 gt- 8 pipelines, 500 mhz core with 1000mhz 128 bit memory.
6800- 12 pipelines, 325 mhz core with 700mhz 256 bit memory.
Processing power wise they are fairly close the 6600 is ~50% faster, the 6800 has 50% more pipelines. I'd call it a wash at stock. Unlock additional pipelines on an agp card and/or overclock and the 6800 can come out a real winner.
Memory wise the 6800 really wins. The 6600 is ~50% faster but the 6800 is twice as wide. You can see this on the link mwmorph posted. Higher resolution and aa/af really stresses the memory bandwidth and the 6800 often shows and definite lead. 30% faster on UT: 2004 at 16x12 aa/af. 22% for halflife at 16x12 aa/af. 50% faster on Doom 3 at 16x12 aa/af. Riddik is 70% faster at 16x12 aa/af. Pacific fighers is 60% faster at 16x12 aa/af. Granted some of those aren't playable even on the 6800, doom 3 and riddik the obvious ones, but for some, like pacific fighters, it pushes you across the 30 fps line and should make a major difference. At lower resolutions and with out aa/af they are typically very close.
If you plan to run at high resolutions and/or run with aa/af on the 6800 definitely has an advantage with it's 256 bit memory interface. Also if your willing to try overclocking and/or unlock pipes the 6800 looks better there. I'd definitely avoid the 6800 le at this point. With the price difference between all 3 being pretty minimal I'd say the 6800 has the advantage right now. The new ati x800 gt is interesting because like the 6800 it's bringing a full 256 bit memory interface down to a much lower pricepoint.