Screw Guru, those buncha dirty Hippies !!
A few recent reviews from Xbit compared GTX480s in SLi to HD5870s in Xfire and a single HD5970;http://www.xbitlabs.com/articles/video/display/radeon-hd5800-3way-cf.htmlhttp://www.xbitlabs.com/articles/video/display/geforce-gtx480-sli.html
Really depends on the game, some a single HD5970 can beat GTX480SLi and others where the HD5970 struggles against a single GTX480. Give you a good overal picture, especially since it includes minimum fps in most tests.
To bad they didn't add dual HD5970, but really that 4th chip (be it ATi or nV) has always struggled in getting good returns over 3.