Actually I got a 'more-cramped' mid-tower case, but it has support for an exhaust and intake fan, at least. The real deal is the fact that I keep my room usually quite cold, especially during the summer with A/C.
I use a cheap $8 CoolerMaster fan, however.
PIII-600E (Coppermine) - 25-28 idle, 30-33 load
Celeron 700@874 - 24-28 idle, 30-35 load
Athlon TBird 1000 - 32-35 idle, 38-43 load
Athlon TBird 1200 - 36-39 idle, 40-45 load
P4 1600@2000 (Willamette, PGA478) - 30-35 idle, 38-43 load
Generally, as power requirements go up, ambient temperature may go up as well, however, even though the P4 3.06 consumes a whopping 82W, the surface temp. is specified never to exceed 42 degrees C, and it pretty much goes for the rest of the P4's despite the intense power requirements. The earlier PII's were hot demon's as well though.