S3 sleep mode does exactly this, it works seemingly perfectly with a computer I built recently: ASRock 939DualSATA2 (AMD64), Antec Smartpower 2.0 PSU and Windows 2000 SP4. All that needed to be done is to choose S3 only in the bios, and set the jumper to +5VSB for the USB stuff. When sleeping every noisemaker turns off. I'd say that any PSU and most modern motherboards can do this.
As for the macs, apparently you cannot install windoze on an intel mac because they don't have BIOS, but some more advanced replacement (that windows doesn't support). Apart from that, if you can live with the lack of games MacOS is far superior than windows IMHO, but Linux is the holy grail