It is a large fan, and just barely fit in my case:
As far as a quad-core, it really only helps as far as longevity is concerned and as a future proofing measure. Games and applications will be utilizing more cores as time goes on. That's the only real reason I'd go to a 940 or 920 over the 720. Also, the 720 is just a quad core cpu with one core disabled because of issues during manufacturing.
$80 more on a GPU wouldn't really get you much better. The HD4890's are not significantly better than the HD 4870 ($60 more), and that's about the next and only higher GPU within the $80 range.