Your PSU is a bit much. Not enough to run the 5850 in CF, but to much for a single card. Might want to drop down abit there as well, or step up to one that can run that CF setup. Either or.
I think that PSU should be enough for Xfiring that card, but not sure though. (I know for sure that it's enough for 2 X 5770's).
If you wanna be sure, you can always go with the Antec TruePower TP-750
On the other hand, if you wanna save some money, go with a OCZ Fatal1ty 550W
if you're not gonna be Xfiring any time soon.