Read the DUC - Digidesign User Conference, at
www.digidesign.com. Those guys really know how to put a computer together. You can't go wrong with Digi's approved list, though some of it's pricey. Forget SCSI - Any new Maxtor, Western Digital, Seagate or IBM IDE drive is more than adequate for pro audio. Seek times of 8 ms or less are perfectly acceptable. (my opinion, of course - your mileage may vary) Just use 1 drive as system drive and another drive for audio tracks.
I run Nuendo on a P4 2.4 ghz (P4TE Motherboard), 512 mb RAM (I find 1 GB ram excessive for most of my needs but, you know, "too much is never enough" , and memory is cheap), 2 - 80 GB Western Digital hard drives, Waves, UAD-1 card, RME Hammerfall Sound card w/ analog expansions. I have no trouble getting 60 -80 audio tracks out of this set-up, more than I need for most any song, even when printing effects.
If you don't want to build your own computer, even Guitar Center sells a computer specifically designed to run ProTools LE/Digi 001 - and it's digi approved. I used LE for 2 years - it's a great program - I just got tired of Digi being so proprietary. And Nuendo is excellent. Read the forums at
www.nuendo.com for a wealth of information.
Definitely use XP as your OS on windows. If you're really patient, stay MAC only and run OSX (or dual boot with OS9.). OSX promises to be incredible over time, but not everything is written for it yet. I would not buy an HP, Gateway, or Sony computer for audio. You'd do much better rolling your own. Especially use RME for in/out, though M Audio products are excellent on a budget. All prices at
www.pricewatch.com
More info on drives at :http://www.seagate.com/support/kb/disc/ultra_ata_faq.html
And, more than you would ever want to know about SCSI, IDE/ATA, Firewire and USB here:http://www.scsi-planet.com/vs/
Listen to the demos for Atmosphere, a Spectrasonics soft synth at
http://www.spectrasonics.net/instrument ... phere.html
This could well change your mind about what virtual instruments are cabable of.
Read a lot and learn a lot before you buy. Have fun, most importantly. We are all very lucky to be alive, playing with all of these fun toys. Speaking of which...