I would go for the HD5850 instead. It OCs to a similar performance while using less power and wouldn't have any crossfire scaling/compatibility issues. It's also a bit cheaper and will leave a slot open for future crossfire(assuming you have a crossfire motherboard.) Also the limited memory subsystem of the HD5770 comes into play more when crossfired which can hurt minimum framerates.
Those are the two options you should consider in that price range however. This would be a good article to read before deciding;http://www.xbitlabs.com/articles/video/display/radeon-hd5770-hd5750-crossfirex.html