It depends. How much are you going to spend on that second 5850?
Crossfire scaling performance isn't that great on 5xxx series cards....it might be just better to upgrade to a single better card and sell your 5850 for some cash to help.
Your PSU should be enough and your mobo supports it though
The reason I ask is because I was in the same position as you a while ago. I wanted a second 5850 off ebay for ~$150 (probably cheaper now). I ended up selling mine and getting a GTX 580 instead for $300 so I basically spent the same amount after selling my 5850, but it was a much better upgrade over 2x5850
Your best bet is to sell your 5850 and get the fastest single graphics card you can get. Crossfire (and SLI) have their issues and its always better to avoid them entirely. I once had 5850s in crossfire and although my 650w Corsair psu was up to the task, I ran into microstuttering and issues with several games.