Depending on your RAM, I would say it is pointless to go above GTX 5series, or ATI 68xx
Even with a GTX 580 you might be slightly CPU bottlenecked, but not by a whole lot.
Yeah, GTX 590 and 690. Also ATI 6990.
Also, EVGA made a custom dual GPU. EVGA 560Ti 2win.
And the grandaddy of all dualGPU's the Asus Mars II