Actually, new hardware has been considered, but with the current price of used Macs it hasn't made much sense to fund the construction of boards that won't offer a performance advantage over what we already have and would cost a great deal more.
As to power use and noise, I have a 1.8Ghz 7447A equipped Quicksilver Powermac which is extremely quiet and has a smaller power supply than my PC.
Of course when G5 support is introduced we;ll have 450 and 600 watt power supplies to contend with and power draw may become an issue.
Right now I'm looking at Freescale's soon to be introduced P5010. It's a single core 64 bit PPC that will run at 2Ghz on introduction. Benchmarks seem to indicate a slight performance lead over the PA6T.
The current version lacks AltiVec, but Freescale has announced that it intends to re-introduce this to the core this processor is based on. This processor uses less than 30 watts.