From what I read you should be ok with that set up. If I understand the article: (http://www.tomshardware.com/reviews/radeon-hd-5870,2422-20.html) correctly the PC that they they tested drew a max of 561W under load, its CPU is also OC @4.0ghz and probably draws more power than yours.
Checking Newegg: (http://www.newegg.com/Store/SubCategory.aspx?SubCategory=343&name=Processors-Desktops) they also recommend at least 600w for crossfire.
I think you may be pressing your luck but from the data I've seen you *should* be able to run both cards. I'm not really the biggest hardware guru though, I'd get a 2nd opinion.