Well in my G5 tower D1.8 here are my readings (15 minutes after waking the G5 up after 15 minutes):
Drive Bay: 87º F
Backside: 89º F
U3 HeatSink: 141ºF
MLB Max6990 AMB: 87ºF
MLB Inlet AMB: 82ºF
CPU A AD7417 AMB: 93ºF
CPU B AD7417 AMB: 93ºF
So how hot is your room? Have you clean the dust out of your Mac on a regular basis? The G5 is dust collector and you have to stay on the top of it.