The 6600GT suffers at higher resolutions with AA on. It's considered a low mid-range card now. The 6800GT still pretty much runs anything with no problems as long as you have a gig of RAM and a good proc. I wouldn't go with SLI in the 6 series only because the 7 series is eventually going to replace it. Your friend might want to look into a 7800GT. You can usually find one for around $400.

What proc do you have, Samuraid?