I've recently found an Nvidia EVGA 690 on Ebay for $679, though I'm wondering if I should just get an 780 for $650 instead.
http://www.ebay.com/itm/EVGA-GeForce-GTX690-4096MB-GDDR5-512bit-Dual-GPU-2xDVI-I-DVI-D-04G-P4-2690-KR/321156720821?_trksid=p2047675.m2109&_trkparms=aid%3D555003%26algo%3DPW.CAT%26ao%3D1%26asc%3D142%26meid%3D193694230440574093%26pid%3D100010%26prg%3D1076%26rk%3D2%26rkt%3D15%26sd%3D140663067488%26
http://www.ebay.com/itm/EVGA-GeForce-GTX690-4096MB-GDDR5-512bit-Dual-GPU-2xDVI-I-DVI-D-04G-P4-2690-KR/321156720821?_trksid=p2047675.m2109&_trkparms=aid%3D555003%26algo%3DPW.CAT%26ao%3D1%26asc%3D142%26meid%3D193694230440574093%26pid%3D100010%26prg%3D1076%26rk%3D2%26rkt%3D15%26sd%3D140663067488%26