I'm building this myself this week or next:
E6750 $223 (processor)
p5k pro epu $155 OR X38 $269 (motherboard)
Ram $74 (2Gig @1066)
Thermaltake Soprano VX w/psupply $119 (case - not sure on this yet though)
8800gt $269 (vid card)
HD 500gig $106 (Hard drive)
Total cost is $1060 with the better motherboard.
I've looked into getting a mac, which is I'm sure what some on here are likely to recommend, but I just can't justify the extra money.
This machine will be pretty fast, especially considering the $1000 price tag.
You could go for the quad core (4 slower cores rather than 2 faster ones) Q6600 processor if you just wanna make tunes - hopefully the program you use will be able to use all of the cores. I play games so I want faster clock speeds (most programs use one core - therefore having four slower cores is no good for playing just one game - but it is good for having four separate programs open at once...). The Q6600 is $275.
You may need some stuff I already have though. This will include a dvd drive ($35) and a good sound card if you wanna make tunes ($200 up if you're serious). You'll also need a display (preferably 2). And you might want to buy a copy of XP if you want a legit one. Or you could get vista. If you get vista you'll want 4 gig of RAM straight away. But don't get vista anyway.
In very simple terms, the motherboard, the processor, the ram and the video card are the bits you need to worry about for a fast computer.
Check these prices:
http://www.msy.com.au/Parts/PARTS.pdf and if you've got more $$$ than me you can go up and up and up in spec to your hearts content.
Hope this helps, I've done heaps of research lately because I'm buying my own.
Upgrades I'd like would be:
Double the ram (will have that within weeks though).
RAID hard disks = 2 or more disks set up to either: mirror each other (good because you won't lose anything if one fucks up) OR you can read data from both disks at the same time (FAST!).
Flat screen monitors x2 (mine are crt's but do the job fine)