Just wanted to report that for those who are interested in running more than 3 monitors in 2010 MacPro, having 5870 + GT120 works fine. I chose this instead of dual 5770s to use the more powerful 5870 (it beats 2x5770s in crossfire as well) for gaming in windows, and be able to do my work in OSX on 4 displays plus stream movies to a tv.
Also wanted thank everyone who helped in the forums with this idea
Also wanted thank everyone who helped in the forums with this idea