i couldn't find anything that says pci voltage speed but i do know that there a 2 different chips in my both graphics cards one has a rv770 and one has m98
is this the reason one is a original ati rv770 and the other is a mobility chip
I know with the 5, 6 and 7000 series you can mix cards (a 7970 with a 7950 can be crossfired as an example). I doubt it matters if the memory is different, it should just run at the speed of the slower model. I would google around for an answer to that though, maybe check with AMD support.
You can mix different RA<M speeds and amounts on 4850s in crossfire they just both run like they are the slower card. The problem is the PSU its an unknown brand priced at half the price of a good 600W PSU.