Requirements for Components:
Case: Must help make the computer silent and keep the components cool. A rack mountable version is a plus.
Power suply: Must be of good quality and match to the components' requirements.
onboard sound: Make sure you can switch it of in the BIOS.
CPU: Look at what you can afford and check the net for compatibility.
Hard disks: One for the system and a fast one for the audio recordings.
Sound cards: Look at what you need and can afford, check the net for compatibility.
Graphics cards: Look at what you really need and check the net for compatibility.