First off.. get a p4. I have several systems with athlon’s.. They are finally solid but it took a long time of swapping parts before everything was finally copasetic. P4's are less hassle and currently faster.

If you’re building your own computer never skimp on the motherboard. THE MOTHERBOARD IS THE ROOT OFF ALL PROBLEMS (potentially). For p4 get an Intel (875 or 865) chipset motherboard rather than the cheaper VIA or SIS based options. Best look at www.tomshardware.com for reviews of current motherboards in the chipset you pick. They do great side by side testing and picking their winners have never led me wrong yet.

latest Intel 875 and 865 motherboard reviews in one article..
Awards: Gigabyte GA-8KNXP (Intel 875P) and MSI Neo 2 (Intel 865PE)

Obviously get a m/board that has firewire built in for the Motu.

All this said.. if you are building your own system your getting a better price over a pre-built machine. However there is always the risk of problems and then your stuck in diagnostics hell. If your not savy with building and/or don't have the time to do so... Look at getting a Dell. Their service (3 years on site) is great. Your computer breaks one day and a guy is at your door the to fix it the next.