Running 3 5970's (6 gpus) on a 1200W psu and overclocking them might be pushing the power supply a bit too far.
Even if it has enough juice, you will need ~80 amps on the +12V rails (or rail) just to keep them running at full load.
Top PSUs are rated for quad crossfire compliance at the most.
They don't even make specifications for 6-8 gpu setups. However I find thermaltake 1500W psus able to run even three 6990's at stock.
80 amps.... is that right?
I was just adding up the watts. my psu is rated to peak at 1300. run 1200 24/7.
each card should pull about 375 * 3 1050. no hard drive or any fans.
I will let them run at stock core clock for a day or so then play with them again.
I must say they cool better then I expected. I have the fan at 60% and it blows the heat out the front of the card
edit: i couldn't wait..... bumped them up to 775/440 while they were running