I have
two perfectly working ASUS GTX570 DirectCU II graphics cards.
Here are the specs:
http://www.asus.com/Graphics_Cards/ENGTX570_DCII2DIS1280MD5/#overviewWork perfect in SLI mode.
note: They are big graphics cards. They will take up 3 bay slots.
$125.00 for each graphics card or $200.00 for both.