It depends a bit on the system specs though. A non-overclocked P4 3.2Ghz with a FX5900, dual 10K RPM raptors in a small midtower represent quite a different scenario than a lower-end system would. The CPU heatsink can make a difference between normal operation and overheating too... as well as your room's temperature (compare chilly winter temps with 36C heatwaves).
In general, two decent fans should be a good bet at a sufficiently cooled system.
BTW If you are going for 1-fan systems, I'd have it on intake, so that the incoming air can be filtered and so that no dust is sucked into every little crack, and also to match the PS exhausting (but I'd make sure there is more CFM going in).