Does it matter to you which manufacturer- No. But i was reading some reviews about the 7970 being a bit loud. I live in army barracks and that may be a problem. Also will the power supply be up to the task?
800W will be more than enough for just a 670. But yes, with a $500 budget for just the GPU, you might as well get a 670 or a 680 if you want; it's about $530. But with a high budget for the GPU, I'm also guessing you're want to spend quite a lot on a new PC. Why not look into building your own?