Why are two videocards better than one? What kind of silly question is that! Two of anything is better than one. One beer: good, two beers: better. One burger: good, two burgers: better. One girlfriend: good two girl... oh wait I guess it doesn't work in that case .
I gotta puss out... I'm a bigger fan of 1 card setups. I guess if you have gobs of money to throw at your rig and bought the best card you could find... 2 would definitely be a plus in terms of performance.
Two are better since GTX 275s in SLI is better than a Radeon 6870 (Not by much but still) so SLI must = better They are also better for folding moar powah = moar science. That and maybe they will keep me warm, it dropped to -2 the other night.