Great build! Wouldn't go for a rackmount tho. However, if you already have a rack or are planning on expanding it, it makes a lot of sense. I have been actually looking at quad socket Opterons, as they have even more multithreaded raw power.
Regarding the resolution question, look at some LTT videos regarding 4K youtube. The thing is, resolution is just a number. What matters more is compression quality and bitrate. LTT, for example, shoots 2160p, edits 1080p, but uploads 2160p because youtube allows for higher bitrate with higher resolution.
TL;DR: Lossless 4:4:4 chroma 720p will propably look better than 4:2:0 5Mb/s h.264 2160p.