if you want top of the line....
hard drives: seagate cheetah 15k-36LP's, a few of those in either a RAID 0 (back up often!) or a RAID 5 (lower performance, but get 20 drives and you wont notice

) not sure what raid controllers are good though.
vid card: geforce 3 Ti 500, shortly gf4's should be out
sound card: if you want it for music (want it to be ACCURATE) then turtle beach santa cruz or for more $$ a Terratac 6fire 24/96 or an aardvark card...if you want it for gaming, then get the audigy platinum, it is less accurate but has way better support for things like EAX which are great for gaming, plus the platinum comes with a neat expansion bay thingie.
cpu: for duallie, you're pretty much stuck with AMD, so a dual AMD XP 2000+ would be nice on a Tyan tiger MP.
ram: as much as you can handle, crucial pc2100 ddr
power supply: pc power and cooling, absolute best quality, but they will cost you.
heatsinks: alpha pal 8045's with some nice quiet 80mm fans like panaflo L1A's.
monitor(s): sony g400's or samsung 900nf's for 19 inchers, i dont know which 21 (or above) inchers are good
speakers: sirocco(sp?) crossfire, i think they're 4.1, either way, beat all the other popular speakers but IIRC they're like 500 bucks