@ tomvertommen
Well, if two 5850 is you choice specially if you buy the factory over clocked ones like the Power Color 5850PCS+ or the Sapphire 5850 Toxic edition ones, then all you can do is sit back, relax and enjoy playing WOW cataclysm or other very high end demanding games without any lags or glitches. Even if you buy two non OC 5850s and manually overclocked it, still you'll enjoy many latest demanding other than WOW cataclysm at very high resolutions.
The difference in performance of a 4850x2 against. two 5850 in Xfire mode is i think around 30-50 percent depending on the games (not sure though, but its close on those ranges) because there's also a factor of FPS increase when using quad core or 6 core processors as compared to high end dual core ones.