looks like ati wants a minimum 550w PSU thats certified crossfire , see the link for their listhttp://ati.amd.com/online/certifiedpsu/index.html
It might just be me but I can't find the 4770 in that list. According to that list a pair of 4890's can run on a 650 W (If I read it correctly that is) so I don't suppose I need anymore than that, but is it safe going down and grab a 550 W ? I'd hate risking anything in my rig :P
I've tried using this Corsair PSU calcualting tool but the 4770 are apparently too new to be included. A pair of 4850 along with my cpu of choice was able to run on a 550 W PSU but do those cards require the same amount of power?
In any case I'd like to be safe in case the PSU ages or I choose to overclock my CPU a little bit.
Link to the calculator http://www.corsair.com/psufinder/default.aspx