I would strongly recommend this TIM
over anything else--especially over AS, which has a cure time. I researched a while ago and I believe this was a few degrees cooler on whatever test rig they used over AS. Tuniq is good, so I've heard, though as well.
As for your case, I don't really like Sunbeam, but cases are really personal purchases IMHO, so you can get what you want. Judging from what Grant explained, it should work for mATX, even though it's not stated. I forgot that the holes still line up...the board is just "shorter".