I'm using three monitors on a single GTX 680 in AH. The CPU is a 4770K with 16GB of memory. I have much more expensive cards, but this is really all you need. You can expand the amount of anisotropic filtering and other features by using later cards, or SLI, but you don't have to do that.
I'm planning on orienting these monitors to portrait mode, because aside from AH I use the system more for digital design (Illustrator, Flash, Photoshop, etc.) and the wide screen format causes too much neck turning.