I only use 6770 since they cost the equivalent of 66.5$ (new with shipping included) where I live.
I undervolt them to 0.8v (stock is 1.125v) so having 8 on a PSU is not a big deal.
But yeah, idk for 7950...
Because 7950 went down in price recently (I get them for $199), so they cost 3x as much as 6770 and at the same time they are 3.5 times more powerful than 6770 (hashing power). In addition I don't have to buy as many motherboards/cpus/memory/OS/hdd to run the same hashes. For 3250Khs (1 rig of 5 cards) I'm only getting one set of motherboard/cpu/memory/os/hdd.
8x6770 would produce about 1600Khs in one rig, right? So you would need 2 sets of hardware to run 3200Khs.
PSU would be the same because $/W of power would be about the same for 6770 and 7950 GPUs.