In Terms of Power Supplies-Try to stay away from discount brand PSU's
I've Built a 2 card 7970 rig with a 850w PSU
3 card 7970 rig with a 1000w PSU
6 card 7970 rig with a 1600w PSU
thats with no overclock?
i wouldnt put my PSU close to full load like that - lowers down the efficiency