[Build Help] Questions about Video Cards

I'm trying to build a pc that can run skyrim maxed out or near maxed out at 1080p. I have two questions.

1. what are the differences between the different brands? There are a ton of different radeon 7850s... Which should I get?

2. I want to spend little and I was planning in getting a Radeon 7850 1 gig. Should I spend more and get a 2 gig one? Or is there no need?

Thanks!