You can get 2 BFG 8800 GTS 512 OC (clocked at 675) cards with a lifetime warranty for around $200 each. I can overclock mine additionally another 100Mhz to 775, without any problems what-so-ever. Could probably go higher, but it cuts through everything I do like a hot knife through butter, so I'll just leave it there.
If you live by a Frys, they have them on sale right now for $199, after a $50 rebate. Retail!