I could be wrong, but I think Crossfire operates differently than a single video card with two GPUs.
Concerning Source games though, yeah they should run fine on it, but I wouldn't expect a massive performance jump as the 4850 you have now probably runs them all near, if not over, the 100 FPS mark. The Source engine is old and still runs single threaded. Only newer games will see benefits from something like Crossfire or SLI.
So to sum it up, the X2 will work just fine with your games, even if it doesn't full utilize both GPU's it may not even need to, depending on the game you are running. It will certainly futureproof your rig for a bit.