Chris, if your PC is not in the same room as the hifi the quietness of operation is not critical (obviously).  You can get various fanless CPU coolers and even fanless PC PSUs (see www.quietpc.co.uk for all things of that type) but IMO it's safer to go with some of the quiet fanned solutions.  Of course if you got an iMac G5 none of this is an issue

btw, I'm not one of those Mac zealots, I use a PC myself but there's no question that Macs are a) gorgeous bits of kit and b) far easier for computer novices to use.  If you have a mate who can build a PC for you and hold your hand through all the wireless music setup then that will be a cheaper way to go though.

Michael.