A workstation was, originally, a single-user computer with more muscle than a
PC, intended to support a demanding technical application, like CAD or complicated
array-based simulations of real world phenomena. The niche that these
computers filled between high end PCs and low end minicomputers has essentially
evaporated. The serious scientific applications have migrated to clusters
of PC’s with near super-computer speeds, and end-user applications, like video
editing, music production and CAD, run well on high-end PC’s. One sector
that still uses large workstation farms from Sun or Silicon Graphics is serious,
Pixar-style animation.
For any of the following uses, you will need the fastest processor and the
most RAM you can manage.

Video editing
Big and fast hard drives are key. 10000 RPM Raptors in Raid 0 as working
space with multiple 750 GB drives or 1 terabyte drives for storage is a good
target. SATA/300 is highly recommended and SCSI subsystems should also
be considered. A large amount of memory (2GB, 3GB, even 4GB) would be
beneficial.

Music production
Plenty of disk space is important, but a music production (recording and mixing)
workstation is chiefly distinguished by specialized external components studio
reference monitors instead of speakers, mixing consoles, microphones, etc. One
piece of advice, if you have extra money, get better microphones, even if you
have to trade the Bluesmobile.

CAD/CAM
More than most, a CAD/CAM workstation tends to be a machine that runs a
single application, so choose your software and build the computer to support
that. Youre spending a lot of money, usually, for your key app, so get advice
from the people who wrote it.