If you have a budget of 750, then in my opinion is a radeon 6950 crossfire is the way to go. you have to keep in mind micro-stuttering. a 3 way set up will almost eliminate ms, however you board doesnt support 3 way cf, (well technicaly it does, but there is no point of running such a good gpu a x4 pcie.) i think the radeon cards are better, eyefinity, support for ANY 3d moniter uvd3 decoder,and the newest catalyst drivers from amd reduced micro-stuttering by quite a bit( it wasnt very noticible b4 ad it no existent now) etc... well thats just my opinion. u might b a nvidia fan and i have nothing against that. the 6950 offers the best price/performance ratio, and are very overclockable. if u get one get it with a good aftermarket cooler and like me u should b able to clock your from 800 mhz core clock to 925 mhz core clock. also some ppl have flashed the bios on the 6950 and turned it into a 6970. turns out the 6950 is just a 6970 with shaders and rops disabled. try finding one with a dual bios switch, the ones that have them u can usually flash, but its been patched on most cards by amd.
This card is great(this is one of the 2 in my pc)http://www.newegg.com/Product/Product.aspx?Item=N82E16814121431
this card stays somewhat cooler but is very big (this is the other one in my pc its enormous but cooler)http://www.newegg.com/Product/Product.aspx?Item=N82E16814121431
hope this helps