There are some nice bits'n'bobs for building quiet multimedia PCs over at quietpc.com. They've got some cool remote control hi-fi seperate type boxes, silent power supplies etc. Wouldn't want your music drowned out by a small hovercraft sounding device!

Well, the plan at the moment is to stream everything over the network, and possibly under-clock the processor, so there might not be any moving parts in the system (flash card rather than hard-drive for the OS, laptop-style power brick...) which will make it quiet without much outlay (that's the theory anyway :-)