what is your psu's model? how many amps does your psu have on it's +12v rail?
i think the 6790 is die-harvested from 6870 gpus. they share the same tdp, only lower specs. right now they are amd's lowest specced 256 bit card. 6790 gfx cards require 2 pcie connectors. they use more power on load (compared to a 6850).
on the other hand, 6850 is the most powerful gfx card that requires one pcie connector. it has lower tdp - 127w. it uses lower power on load (compared to 6790) but performs better than a 6790. lower power consumption also means less heat, less heat means less noise as the cooler has to work less.
btw, both gpus are built on 40 nm fabrication process.
now it comes down to brand. i don't know much about powercolor...google it's review. but strictly in terms of price and general performance, the 6850 is better.
here's an excerpt from the 6790 review: