its not about the motherboard its about whats connected to the motherboard. usually high-end cards need a lot of juice, so thats where you'd need a good PSU. also in the case of SLI, Crossfire, or in general if you have a lot of components...
for your build, the 450watt more than adequate. if you had a higher end card then maybe you'd need higher.. but with the 6600 you don't have anything thats juice-hungry (or thirsty..)
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius