The 7770 and the 6850 perform similiarly with the 6850 coming out slightly ahead. The kicker is the 7770 uses a lot less power than the 6850.
I'm loving my wizard btw... he has an awesome attitude problem... so vain...
You should get the 6870. 7770 is lower power, yes, but it's much worse performance. The problem is that AMD has no 7000 series card right now that fits in the 6870's niche. And given its discount, the 6870 is a great value card.
in my experience, noise is often directly related to design and heat, since the design of any non-reference heatsink will differ, it will come down to heat, and since a card that uses more juice runs hotter, the 7770 will generally be a quieter card, but the 6870 will be a far better performer.