since the 6770 is a rebranded 5770, it can be CF with a 5750. the 7770 is a 28nm process as opposed to the 6770/5770 40nm. so cf a 7770 with the 5750 is out of the questionhttp://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg
the 7770 is $130-140, while the cheapest 6770 is $110. CF 6770 would have similar performance to a 6950, so a cf 6770/5750 would have performance similar to (please correct me if I'm wrong) a 6850/6870 (?). a 7770 performance is pretty close to a 6850. In that sense, I would get the 7770.