Your board should run 2 x 5850 at 8x/8x which is a small hit but still faster then a single 480GTX and would use alittle less power then the Nvidia card.. the best thing about CrossFire is in 2D you only use one card so the power draw is min and once you fire up a 3D app then the second card wakes up so your not pulling the total power of 2x5850 but only when needed.
Also you can find used 5850's around $225-250 which would save you enough money to try Eyefinity if you can find some cheap LCD's.. I run BC 2 at 5760 x 1080 in high settings with my CX 5850's on an Evga x58 e758-A1 board and the i7 930 cpu and it's a force in surround gaming..
I would suggest you overclock that HD5850 as much as you can, If you can get it to 1000+ Core and 1200+ Mem It'll be slightly faster than a stock HD5870 and very close to a GTX480, However you would need some good cooling so either something like this: http://www.overclockers.co.uk/showproduct.php?prodid=HS...
Or a full blown water loop.
Thats my suggestion, I guess if you want the advantages of an nVidia card like PhysX and Cuda and you can get a good price for your HD5850 then by all means go for it.