Just giving you guys some ideas...
I build that "HTPC" for my living room to watch movies/listen to music/satellite DVB-S
But its still a good system for music production.
Advantages: Consumes only 50-60W (saves you money on your electrical bill), lightweight and compact, enough processing power. Built in graphics card.
The whole thing costs only ~ 420$
Additional stuff (not everyone needs those)
- Keyboard Mouse (30$)
- OS (Xp,Vista or 7 ~100$)
- Professional soundcard (~100$)
You can always use your mouse/keyboard, OS and soundcard from your old PC.
Here are some pictures: