Well, most of the vanilla 6600s with 256 MBs of ram use slower 500Mhz ram, while the 6800 with 128 MB use slightly faster 700Mhz ram. Additionally, the 6600's use a 128-bit memory bus while the 6800's use a 256-but memory bus. The 6800 therefore has less memory, but has considerably faster access to it.
This is of course refering to the non-gt versions of both cards. The gt versions both use fast GDDR3 ram, but the 6800gt has the 256 bit bus while the 6600gt has the 128 bit bus, meaning the 6800gt again has faster access to memory.
As for SLI, well, it's largely a matter of what resolution/AA/AF you want to play at. If you're playing on a 15" LCD limited to 1280x1024 resolution, you're unlikely to see any real advantage even with 8x/16x AA/AF. On the other hand, if you're playing at 2048x1536/8x/16x on a 21" CRT, you'll definitely notice the difference.