there is a "new" (well not so new) PC concept called SFF (Small Form Factor). I am sure you have seen them somewhere. They have very small cases, with special motherboards and crammed components inside, that provide all the functionality of a tower PC but at a much smaller size, and are usually very elegant and pleasing to the eye. They are supposed to be sitting in the living room, along with the TV or Hifi, so the appearance is well thought. Some of them come with a handle too... (gigging, anyone?)
They usally have 2 slots for add on cards, one for a better graphics card and another for a PCI card, (the badass souncard goes there!) and usually have everything else onboard, like 4 or 6 USB, modem, network, TV out, 6 channel sound card, firewire, etc.
I think those provide the best source material for building a dedicated music computer. They are compact, can be carried around easily, have the looks. A drawback may be that some are noisier than average, but you can find quiet ones, carefully choosing the components. They also don't offer much room for expansion, but, If you do your maths and your thinking beforehand, you can get one that will serve you well.
Take a look, (some models shown may be older, but you can get the picture) and tell me what you think. http://www6.tomshardware.com/howto/200502161/index.html http://www6.tomshardware.com/howto/20040804/index.html