Oh hell no. You have a perfect MOBO for CrossFire, you can even pull CFX.
If you want the most out of your MOBO, an HD5970 + HD5870 trifire is the best.
What I would do though, like I posted above, is to OC an HD5850 to 950/1250 core/mem, then Crossfire another one later. You'll get massive performance.
lmao, Don't think the 5970 ($700) and 2 5870s (~$430 each) will be happening anytime soon, unless I live off of Smiles and Happy thoughts :P
I'll look at the 5850, the main attraction to the Nvidia 4xx was the PhysX, otherwise I see no real reason to get it. Also, if I get a 5850 can I crossfire it with my current 5750?