The central fact about clone hardware that conditions every aspect of buying it is this: more than anywhere else in the industry, de-facto hardware standards have created a commodity market with low entry barriers, lots of competitive pressure, and volume high enough to amortize a lot of development on the cheap.
The result is that this hardware gives you lots of bang-per-buck, and it's getting both cheaper and better all the time. Furthermore, margins are thin enough that vendors have to be lean, hungry, and very responsive to the market to survive. You can take advantage of this, but it does mean that much of the info in the rest of this document will be stale in three months and completely obsolete in six.
One good general piece of advice is that you should avoid both the highest-end new-technology systems (those not yet shipping in volume) and the very cheapest systems put out by vendors competing primarily on price. The problem with the high end is that it usually carries a hefty ``prestige'' price premium, and may be a bit less reliable on average because the technology hasn't been through a lot of test/improve cycles. The problem with the low end is that price-cutters sometimes settle for marginal components. Unix works your hardware more efficiently than DOS or Windows, so it is more sensitive to hardware flakiness, which means cut-price systems that might deliver consistently for DOS/Windows lemmings can come around and bite you. Use a little care, and spend the $200-$300 to stay out of the basement. The avoided time and hassles will be worth it.
The last point deserves a little amplification. In the PC world, there's a lot of ``if it doesn't fail, it's OK''. It is common to ignore normal engineering tolerances (allowances for variations in components, temperature, voltage margins, and the like) and to assume that anything which doesn't fail outright must work. Watch out! As a historical example, the ISA bus was originally designed for 6 MHz. IBM later updated that to 8 MHz, and that's as much of a standard as there is, yet there were motherboards that will let you (try to!) run it at 12 MHz, 50% over spec. Some cards were actually designed to work at that speed with proper tolerances. Others might work...or they might flake out when they get warm. Any systems vendor above the fly-by-night level is going to shoot for a little more reliability than this, burning in systems and (often) doing at least a token system test with some kind of Unix (usually Linux these days). Pay the few extra bucks it costs to deal with a more careful vendor.
The happy bottom line is this: at July 2001 direct-mail prices, you can expect to get an AMD K6 or Pentium III 450 system with 64MB of memory, 9gig EIDE hard disk, 3.5 floppy, 101-key keyboard, 32X CD-ROM drive, sound card & speakers, SuperVGA-compatible 17" monitor, 56KB modem, and a decent AGP video card for $800 or even less. This is a more than reasonable Unix and X machine.
I put together the first version of this guide around 1992; Unix-capable systems are now five to ten times cheaper than they were then. At today's prices, building your own system from parts no longer makes much sense at all -- so this HOWTO is now more oriented towards helping you configure a whole system from a single vendor.