I use a GTX 680 4GB FTW. If all you're trying to do is run it though, you can get away with a GTX 650 or maybe even a GT 640, just don't expect to do any gaming. If you want to game, you'll want at least a 670 if you want to actually play at any respectable settings. Dual-Link DVI is just the interface being used. You'll want a graphics card that has 2x Dual-Link DVI ports on it. Single-Link will NOT work. So no ATI unless you're willing to spend a little more for the one with multiple inputs (HDMI, DP).
thanks, great info. It's only for photo/video/software development. I run portrait orientation, so not much can be used for gaming.
Windows 7 x64 = Best HTPC software