1) No, the HD 7850 is superior. And this is a better card
, in case you decide to upgrade your monitor down the line. (You'll be hitting 1GB+ of VRAM usage playing the most intensive games on Ultra at 900p.)
2) No, no reason to.
3) Yes, 500W is plenty for that build, and GPU power usage is going down this gen.
4) No, if it's listed you can buy it.
5) Looks like a good build. I have the same res, and I can max out any game on ultra at 40fps or better.