For those with G5s, beware of the dust. Unfortunately, just blowing out the inside may not be enough, you may eventually have to pull out the processors, heatsinks and the bottom plate to get to the power supply.
This was posted to xlr8yourmac.com in late September:
http://www.hatsuon.net/macstuff/G5_TAKE_APART/Disassemble_G5_DP_2Ghz.html
Holly cow! That is a very nasty dust problem. I forgot about the dust issues with desktop units as it has more vents and openings to introduce dusts inside the machine.
Maybe Apple should put in a air filter behind the front panel as the fans suck air from the front to the back. That would serve as the air cleaner and dust prevention.