Some important points to remember.
75w and 150w are just the minimum requirements that PSU's have to be able to provide their 6pin and 8pin, not the maximum. 75w and 150w is the maximum that GPU's are allowed to ask from 6pin and 8pin PCI-E power, not the minimum.
Like nightfox pointed out, what matters how many watts you get is your +12v rail amps and number of +12V rails and wire gauge. Although from what I can tell, your PSU has 2 +12V rails
M12II 620 Bronze
Output +3.3V@24A, +5V@24A, +12V1@24A, +12V2@24A, -12V@0.8A, +5VSB@2.5A
with each PCI-E almost certainly being on a separate +12V rails. This means they are being fed the same power and only have different connectors on the end. 24a x 12v is 288w per rail.
AFAIK, 6+2 pin is nothing more than extra ground wires jumped out so that higher wattage GPU can have parallel ground circuits internally. Operative word being can. Just cause it has an 8pin socket, doesn't mean it does.
EDIT: I just read that 7970 requires 23a.... and you have two 24a rails.... this means that you could likely power the card with just the 6+2 connector alone. I wouldn't lose any sleep over having a 6+2 and 6 pin connected, even to an OC model, but if you do lose som sleep, you could get the 6 pin to 8pin adapter that nightfox linked. I would feel safer that then molex to 8pin becasue you don't know if the molex is sharing the same rail with the 6+2 pin or the 6 pin, you certainly don't want that if it's on the same rail as the 6+2 pin
Edited by WOD35 - 8/20/13 at 5:56pm