I have 3 gpu's running in orac, and 3 gpu's running in caro. caro does have a free slot but I can't get a gpu to run successfully in it. I did have 4 gtx 660 ti's in it at one point, but couldn't keep the system stable. I suspect the asus mobo can't provide enough power to the 4 pcie slots when they are running flat out on Boinc. Have shipped some ribbon cables over from Hong Kong with the intention of building some powered risers which hopefully will sort this out. Then for a while orac will have a gtx 650 running in it until another 7970 finds it way onboard and caro will have the 4 gtx 660ti's.
Guess after that I have to build Aroc
but Orca does want some more android droids as well
Will have to look into the new nVidia and ATI cards but I suspect at the moment £ / crunch the 7970 is still the best with the 660ti still 2nd. the newest cards are probably best with single card systems but not when you start running multiple cards, as the newer cards have far higher power requirements,the benefits are not reflected in the increased output.