if you are limited with power and space dual gpu cards like the 6990 would be ideal as you can fit them in a full size ATX tower and still have 8 GPUs to play with. however, with the proper server chassis, 8x6970 (which is what the 6990 is based on) will be faster than 4x6990, and 8x7970 is faster than that.
there are a couple members here who have 8x7970 in a single server.
https://hashcat.net/forum/thread-1515.html
http://www.tmto.org/docs/HardwareComparisonGTXvsHD.pdf
http://ob-security.info/
note: in a single system, AMD drivers are limited to a max of 8 GPUs where Nvidia allows max of 16 GPUs
also, powercolor just announced their new dual gpu card based on the 7970 and should be released soon.
http://www.powercolor.com/global/product...asp?id=423
there are a couple members here who have 8x7970 in a single server.
https://hashcat.net/forum/thread-1515.html
http://www.tmto.org/docs/HardwareComparisonGTXvsHD.pdf
http://ob-security.info/
note: in a single system, AMD drivers are limited to a max of 8 GPUs where Nvidia allows max of 16 GPUs
also, powercolor just announced their new dual gpu card based on the 7970 and should be released soon.
http://www.powercolor.com/global/product...asp?id=423