Rosewill RP600V2-S-SL 600W work with 4850 X2?

If you've not already bought it, that Rosewill PSU is an older, inefficient design. Some may accuse it of being garbage, but that I don't know, because a lesser model in that line did pass jonnyguru's testing. If you have it already, it should be enough, but the margin is likely pretty slim. If you don't already have it, an Antec Truepower New 550W PSU would work well for you.

If you have not yet bought it, then I would strongly urge you not to. There are too many remarkably better PSUs out there. At a minimum, look for full range Active PFC (no little voltage switch) and 80+ certification (or better; the Truepower New is 80+ bronze). The 80plus program tests at 20%, 50%, and 100% load. If a PSU passed, it means not only could it produce its labeled output (vs, for example, a Chokemax, which explodes at 75% load), it could do so efficiently.

+1 for not getting the ROSEWILL. Like said go Corsair, PC Power & Cooling, and Seasonic, Antec, or OCZ.

I'm assuming you are going with the the 4850X2 (Dual GPU card) and not CrossFire? Please clear this up.

@OP: List full specs and budget.

Unlocking the 4th core is not a sure thing. Assuming you get a mobo that still has a BIOS that can do it, the core may or may not be stable. YMMV. My 720BE only "sort of" unlocks, and I haven't yet taken the time to tweak it to see how stable I can get it. With a Q9450 as my primary, it hasn't been a priority.