I'm a huge cooling freak, with my aspire x-dreamer II loaded with 11 fans that can be louder than my vacuum cleaner (6700RPM cpu fan? what?)
I highly recommend big airy cases with lots of fan holes, especially those with chimney fans.
I also highly recommend the aspire x-dreamer and x-navigator case, which you can purchase for around $50 (x-dreamer) to $150 (x-navigator with 120mm side fan)
I can get colling numbers round 32 overclocked to 2.6GHz with my athlon xp 2800+
a case is one of the most inportant parts of a pc.
but of course, the pc with the best cooling are those without a case.
ahahaha
so if you want cooling (and aren't going anywhere with your pc and don't fear damage to it) just take the entire assembly out of teh case, set it on a desk or shelf, and power it up.