I run 2 gtx 275s in sli.
As said in previous posts expect about a 50% increase in framerates.
Battlefield 3 is double the fps from my own testing, think i was getting around 45 fps on ultra with aa turned down a notch. Elder scrolls is very cpu dependent and my brother sees better framerates than me with 2xgtx 260 in sli, 2.8 vs 3.8 phenom processor, i still get 40 fps in gpu intensive areas and 60 fps in dungeons.
As you probably already know these card run hot, not sure what kind of gap you have between your gpus in sli on that mobo, but i would reccomend a double wide space slot. Also a good brand 700 watt min psu.
Worth getting if its cheap, say 70 dollars