I have my custom made PC with Prescott and 2 GB RAM for 2 years now - works good enough to make my demos done (Mbox - PT 6.7 LE, digidesign plugins, third party plugins).
Care is required for picking up quiet components. Avoid very high end video cards due to the noise. My Prescott needed custom cooling system to keep it cool enough and quiet at the same time.Memory management: 1 fast HD where the recording goes and 2 with higher capacity. Add some usb ports - never enough.
Any computer shop would assemble for nominal cost. Don't save on RAM and motherboard
And my PC kicks my neighbor Mac ass!!! ( still would prefer Macs - if budget allows, but it is rather political)