Compaq has a history of building machines with proprietary components, and folks like Dell use their own motherboards, which makes fixing them a pain and expensive if you don't buy service.
So I prefer machines with more vanilla components, ones that I can dash down to Fry's and replace if they fry (bad pun, sorry).
I should write a FAQ on this, but unfortunately hardware recommendations have to be updated frequently.
Here's a site that has tons and tons of information on components, etc: http://www.tdl.com/~netex. Click on "motherboards" and you'll get a list of motherboards with features and prices and they only carry parts that are good quality. In that list, click on the part name and you'll be sent to the manufacturer's web page on the part.
Very useful.
If you just want to run a moderately busy personal site or the like, any 'ole celeron, 128MB or 256MB RAM, a couple of UDMA disks forced to run in DMA mode (Linux usually defaults to PIO), a decent DEC Tulip-based or Intel or 3Com NIC and your on your way, no sweat.
Or buy a low-end machine from VA Linux or the like (or NetExpress, the site I listed above, though they seem pretty much booked these days).
