I have given this advice for many years.
1) Chip & Motherboard & Chipset – The fastest, biggest, most advanced you can possibly get. Look for 1394, eSATA, SATA & lots of USB headers on the MB.
2) RAM – Enough to use, you can always add more (if you get #1)
3) Hard Drive – Enough to use, you can always add more (if you get #1)
4) Video & Sound cards – Enough to use, you can always get better ones (if you get #1)
5) Case – Big – with lots of expansion space.
6) Power supply – Big enough – You can always get a bigger one if need be.
7) OS – Whatever you’d like to use for the life of the machine (upgrading OS is a royal pain – last time I did so without problems was NT 3.51 to NT 4 and even then there were bumps).
Bottom line – figure out your budget and then spend 60% of it on #1 (assuming you already have a decent monitor), leaving 40% for all the rest.
That’s why I still have a usable six year old Pentium IV machine today that runs Vegas Pro 8 without problems.
