What are you people talking about, one 6600 gt and 2 6600gts. From the lists of the two systems they are 6800 GT's! NOT 6600gts. There is a huge difference there.
Personally I think this shop is giving you an excellent deal on your system. Definitely purchase the SLI system from them. You are paying about the same you would if you bought it yourself IF NOT EVEN LESS.
If they have some kind of warranty on their computers or good service then don't pass this up.