The only thing I'd add regarding my setup is that I initially went with 3 x Gigabyte 7970 cards but sadly came to the conclusion that since they were more recent models, I can't get the hashrate of earlier models (~750) and only ended up with ~550. It has to do with the most recent BIOS those cards have and you can't flash back to earlier versions, I've tried several. Luckily I ordered from Amazon and they're great about returns so they're going back and I'm replacing them with 4 x 270x. Not as good as yours but with 4 of them I'll get a better hashrate overall than my crippled 7970's
What version bios you using? Is it possible to mod the bios you having using something like VBE7???