I'm running a 6670 on a 400w PSU, +12v 10a = 120watts (Dual Rail). Never had some issue.
6670 TDP - 66w
7750 TDP - 55w
I have a small question, I think Mobo's are using +3.5v/+5v Rail?
Can you tell me your PSU model?
Also, motherboards do use some 3.3V and 5V. As to how much, I'm not sure. I've read that motherboards tend to use 20-40W of power, but I'm not sure of the ratio of voltages that are involved and I'm not even sure of how well that applies to current motherboards.