Creative Sound Blaster X-Fi Xtreme Fatal1ty
Kingston 4gb DDR2 800mhz/PC2-6400
AMD Phenom 9500 Quad Core AM2+ 4MB Cache 2200MHz
MSI K9A2 CF-F Socket AM2+ 790X DDR2 ATX Motherboard
Seagate ST3500320AS 500GB Hard Drive SATA II 7200rpm *32MB Cache*
3dInno 256mb 8600GT graphics
That sort of spec is what I would recommend for the above applications.
4gb if using Vista...2gb if XP.
As fast a processor core as you can afford as music applications are very processor intensive.
A good dedicated sound card for the above reasons.
An average speed graphics card just as they are pretty cheap under £60.
And 500gb HD as you can never have enough room for files and the difference between 250gb and 500gb is negligable.
Nice easy build for you Si, about 2 hours work including installation of OS.....sweet.

And all easily under £500