Personally I would get the 5850 instead of two 4850s. Better power consumption, cooler, Direct X 11, very powerful and you can XFire in future. At a cost of about $50 (post rebate) more than 2 4850s it is well worth getting
The 5850 it offers great performance but the 5770 is also great. Also if you want to Xfire two 5770s they will give about the same performance as the 5850 but if the game does not use Xfire it will run off one cad.. But did you think to go AMD and go with a 5870?