GPU recommendations?

So I've got this Alienware m18x with dual ATI HD 6900M cards in Crossfire, but they just kicked the bucket on me. Anyone know which graphics cards this beast actually supports?
If you could drop a link, that’d be awesome!

http://en.community.dell.com/owners-...746/t/19538866
http://www.tomsguide.com/answers/id-...u-upgrade.html
https://eu.alienwarearena.com/forums...graphics-cards

http://www.hidevolution.com/dual-nvi...d-m18x-r2.html