A few thoughts:
Monitors: If you are looking at LCDs be sure they are DIGITAL (have a "DVI" input) and not analog-only. Also, unlike CRTs, LCDs have a "native" resolution where they should be run. Most 15-in. are 1024x768; 17-in. are generally 1280x1024. After size and resolution, look at dot-pitch (smaller=better) as well as response time (<35ms).
Pentium 4 Processors: Be sure whichever you choose employs the Socket478 and not Socket432. '478 is newer and will be forwardly compatible with faster CPUs.
PC Memory: As suggested earlier, get PC800. In general, for our purposes, as a broad rule of thumb, at this time, memory over 512MB is not that useful.
Video Cards: Go for AGP (opposed to PCI) on a new system and make sure it has a true "DVI" output (for your digital LCD). All the 3D, TV-out, Gaming stuff currently popular has little use for us. Matrox is my card of choice, for no other reason than I like them.
Hard Drives: Unless you have a real reason to go with SCSI (faster and more expense) be sure to shop a system with "Fast ATA 100" interface and mate it with an appropriate 7200rpm ATA 100 drive. (The difference in cost between 10gb and 40gb is generally less than $50 for the same drive.)
I used to be a "build your own" but the world has gotten much more complicated. Instead I went "off the shelf:"
Dell 8200, 1.9Ghz P-4
512MB PC800 memory
40gb 7200rpm ATA 100 hard drive (WD caviar)
NEC 16x/10x/40x CDR/W
Windows XP Professional
All other options (sound, video, keyboard, etc.) were the
cheapest Dell offered, as I never planned on using 'em.
-- $1200
Added the following:
Logitech iFreedom Cordless keyboard + Optical Mouse
Belkin 600VA uninterrupted power supply
Matrox G550 Dual DVI
SGI 1600sw Flat Panel (one of hopefully two)
The construction of the Dell box was good, easy to upgrade, pretty quiet. No problems as of yet. I opted for the 1-year on-site warranty on the basis that it is either going to work or not within a short period of time. Dunno about XP as yet. I was a staunch supporter of W2K, but went for XP in this deal because of price.
HTH