Ok, I'll give some advice with an assumption - You mentioned he's a DJ because that's what he's going to be using the computer for, not gaming or creating content.
Case - personal preference, most people seem to agree that antec and enermax make the best PSUs. Here's one, but go with whatever look he likes best.
http://www.newegg.com/app/View...=11-124-106&depa=1
$55.00 + $13.99 ship
Motherboard - Socket 939 - it's good performance for the dollar, and will be very upgradeable in the future.
http://www.newegg.com/app/View...=13-180-064&depa=1
$114.00 + $5.00 ship
Processor - AMD64 3000+ - you don't need a speed demon, but this, again will be upgradeable
http://www.newegg.com/app/View...=19-103-486&depa=1
$151.00 + $0.99 ship
Memory - 1 gig of pc3200 should be plenty
http://www.newegg.com/app/View...=20-145-440&depa=1
$156.00
Hard Disk - 200 GB should be plenty, unless he's going to store a lot of large files, in which case, just get another hard disk.
http://www.newegg.com/app/view...=22-148-032&DEPA=1
$108 + $2.75 ship
Optical Drives - I would do a seperate DVD ROM and burner. They're relatively cheap anyway.
http://www.newegg.com/app/View...=27-130-027&depa=0
http://www.newegg.com/app/View...=27-130-020&depa=0
$20.99 + $4.00 ship
$54.00 + $4.00 ship
Operating System:
XP Home. Make sure you buy it OEM when you buy your motherboard.
http://www.newegg.com/app/view...=37-102-151&DEPA=6
$87.00
The price of this system before soundcard and software is 776.72 at newegg. If you shop around, you should be able to get the components a little cheaper.
Really the two most important parts of the system will be the sound card and the software, and I donn't know all that much about either one for DJs, so you're going to have to look around. Here's a couple links I found, but you're going to have to do some research.
Software:
http://www.mixvibes.com/
http://www.mixmeister.com/
Sound Card:
http://www.tracertek.com/canam.htm