if you have res you'll want to put it before the pump so it'll prime itself. just do whatever you gives you the shortest/easiest tubing for the rest of it.
It's actually not *that* big a deal if pull air from inside the case through the rad. Since you've already moved the major heat sources into the loop the air inside won't be much warmer than ambient.
that radiator is junk. you'll have more noise for less cooling with that. i can't believe anyone who reads this forum would buy an 80mm rad, dual or no. just do the math on surface area for a 120mm rad versus surface area on a dual 80mm rad. also, since the heatercore would be cheaper you'll...