|The Unix Hardware Buyer HOWTO|
The PC Tech Guide offers pretty comprehensive descriptions of PC hardware technologies.
The Caveat Emptor guide has an especially good section on evaluating monitor specifications.
Dick Perron has a PC Hardware Links page. There is lots and lots of good technical stuff linked to here. Power On Self Test codes, manufacturer address lists, common fixes, hard disk interface primer, etc.
Anthony Olszewski's Assembling A PC is an excellent guide to the perplexed. Not Linux-specific. If you're specifically changing a motherboard, see the Installing a Motherboard page. This one even has a Linux note.
Tom's Hardware Guide covers many hardware issues exhaustively. It is especially good about CPU chips and motherboards. Full of ads and slow-loading graphics, though.
The System Optimization Site has many links to other worthwhile sites for hardware buyers.
Jeff Moe has a Build Your Own PC page. It's more oriented towards building from parts than this one. Less technical depth in most areas, but better coverage of some including RAM, soundcards and motherboard installation. Features nifty and helpful graphics, one of the better graphics-intensive pages I've seen. However, the hardware-selection advice is out of date.
The Linux Hardware Database provides, among other things (e.g., drivers, specs, links, etc.), user ratings for specific hardware components for use under Linux. Our ratings take a lot of the guess work out of choosing which hardware to buy for a Linux box. The site also provides several product-specific resources (i.e., drivers, workarounds, how-to) that help users get hardware working after they have made a purchase.