You should basically get the same performance but on the other hand....
Think about the simplicity of having one card, one heat source, and a free slot to upgrade to CrossFireX (with a 4850 1GB) or CrossFire (with another 4850 X2).
Running a single video card is a "risk" that a LOT of people take. LOL. If it dies, you'll be force to buy a new one anyway. Not an issue in my opinion.