I'm a Network Administrator for my day job, and built a new computer specifically for writing music earlier this year.
I'd recommend giving
www.scan.co.uk a call. Ask for a guy called Paul Moss and tell him Luke from Aberdeen sent you. Tell him you're interested in having a music computer designed. He'll sort you out.
Beyond that, here's a few things to consider:
1. Do you need the computer to be quiet? Only truly important if you are going to be recording in the same room.
2. Are you likely to use vast swathes of samples? If so, you may need to consider getting a 64 bit computer. As VC doesn't work in 64 bit OS's, you might want to consider getting a "main" DAW in 32bit to drive your Virus and your sequencing software, then farm out all the sample processing and VSTs to a seperate, 64 bit DAW - and then join the two together with something like VST System Link (in Cubase).
You cannot get more than about 2.5 gigs of RAM in a 32 bit computer.
Hence having a 64bit box with loads of memory for your samples/VSTs.
3. Will you be mounting it in a rack? You can get rack-mountable computer cases (like mine!)
4. What Operating System will you use? I'd recommend Windows XP Pro (NOT Home Edition). It is very stable and good for music writing.
Specs of my DAW:
<10db operational noise
Quad-Core 2.15GHz
4gig RAM - 2.7 useable
random A-Bit motherboard
Fanless GFX card
Drive Noise Dampeners
120GB Solid State OS Drive
2 x 750GB drives in Raid 1 Array
Cubase 5 Studio Edition
Onyx 400F
2-port DVI/USB KVM Switch
Funky Blue Lights To Make It Look Awesome ^_^
Price including Assembly, Tax, Delivery: £2,500.00
It's in a giant black rackmountable metal case. The whole thing is so full of heat sinks (instead of fans) that it weighs around 40kg. Quite tricky to lift it.. :>