you should go for the 6600GT... it has more "bang for the buck"
it also has a higher core and memory clock and can probably be oced too. the main differences between the two are that one is 128 bit and one is 256 bit. I don't think you will really see much peformance boost with the 6800 so i say go for the 6600GT. If you decide on SLI go for the dual 6600GT's
ALSO NOTE: the 6800 says it's memory type is only DDR... and the 6600GT says its memory type is GDDR3(i bit of an improvement)