Myself, I'd wait and save for the 7800GT. It's going to offer you more performance than the SLI 6600GT's will and when you find that the games are getting to be a bit much for the single 7800GT (should take a while there) you can always opt for another one and run the 7800GT's in SLI whereas with the 6600GT SLI setup you'll have to scrap it and start over which means you'll be throwing even more money into the eqation.
Comments
Just my two cents worth.
Techreport 6600GT SLI article