It would be possible but I would go for ATI 4870, since ASUS Rampage Extreme supports ATI CrossFire™ Technology and NOT SLI.
But if you only wont ONE graphics card installed it is possible to use the xfx 280 gtx.
With some hack NVIDIA (NOT OFICIAL DRIVERS) it could maybe also be possible to run SLI.
ASUS Rampage Extreme - Review 1http://www.legionhardware.com/document.php?id=763&p=0
ASUS Rampage Extreme - Review 2http://www.elitebastards.com/cms/index. ... &Itemid=27