Video: 2 x EVGA 256-P2-N636-AR GeForce 7950 GT (~$400 for both cards)
... a better option than this would be a GeForce 7950 GX2... it only needs one pcie slot and takes all the data transfer load off the motherboard by being a 2-in-1 card. (you can run it as 2 cards or 1 really powerful card).