I actually changed now and it is between the slk3800b and the sonataII. I can get the sonata II for 108 and the slk3800b for 80. The 3800 has a little smaller psu but not much. It has more size so easier to cool and one more 5.25 bay.
my quietest would be the powerbook. My tower is pretty silent when its cool in the room. But when those fans kick in full blast, I can hear the hum downstairs. I'm so accustomed to the hum of a fan, I never cared much for making them silent.
less noise means less airflow, which means more heat.
Not neccessarily. I've seen cases that made no sound at all and kept the PC very cool. Used heat pipes and the whole case acted as a sort of heatsink for the whole system. The downside, the case costs about $1k
Well you could make the case your self, i have seen many good cases made from many strange objects just do a google search. Some things can make it quite e.g. making it from something with thick metal or something. E.g. Toaster :)