Brad,
I would recommend Nvidia, they have been my choice for years.
I have an i7 system at work I use to test out all the latest and greatest stuff that comes down the pipe. It has 24 GB of RAM, the i7 is 3.2 GHz (960, not a sandy bridge) I have 2 GTX-460’s by ASUS, It does very well (until I compare it to my main machine.)
One rule I live by is your MB and video card are by the same manufacturer. My test system has an Asus MB and and 2 Asus video cards. My main system has an EVGA MB that I put an EVGA video card in. I might be wrong about this, but I’m one of the few here who has not (so far) be put through the wringer over stability issues.
Just my 2 cents worth,
Jeff
2-Xeon X5680 @ 3.33, EVGA SR-2 Mobo, 48GB DDR3, GTX 580 3072MB, 16TB Attached Storage, Win7, Vegas 11 x64