This modder took a very scientific approach to reducing the heat in his G4. Using two sensors in two different spots inside the case, over a 7.5 hour time period, the modder charted 7 different graphs. This includes charting the CPU fans and the PSU fans; as well as 3 other 3rd party fans to replace the CPU and PSU fans. Interesting info.

http://g4noise.com/mods.php?ID=53&p=all