I wouldn't bother with crossfire for the performance class you are looking at. A single card is far less trouble.
As far as 5850 vs GTX470. They are both good cards. The GTX470 beats out the 5850 more so than not. It does pump out more heat and use more power. Your power supply would handle either. They are both good overclockers as well. So it is kind of your call, but those look like your best options.
The GTX 460 is out there for $230 and is also a good card that uses less heat and really performs well. It hangs in there slightly below the 5850.