The 600w OCZ will be fine if you do no plan on crossfire. The 5850 will fit in the storm sniper. The storm sniper is horribly overpriced for a mid tower case though, look at the Coolermaster 922 or the Lian Li PC-K62. The 5850 will also fit in both of those cases. Be warned you will most likely have to buy a 8 pin power extender if you get the 600w modxtreme as it is only long enough to reach the top of the mobo if it goes over top of the video card on top of the the mobo which is not good. The extender is only like 7 dollars though. Look at the Spinpoint F3 HDD, it is faster and cheaper per gb.
Erm I had some mistakes there. Yes the Storm Scout should also be fine on size. I meant to type over top of the videocard on top of the motherboard. Don't think so on the stealth since it isn't modular.