It is amusing to consider these questions -- these days, you can get great performance on almost anything you throw together, and for cheap. The style of PC construction prevalent a decade is almost obsessive navel gazing now. You just need to focus on the mainboard for stability, and snooze the rest.
I shuffled through a number of boards for my primary box, after UPS gave me the impetus by destroying my machine. I've settled on Epox - Tyan & ABit are as stable, but Epox gave me more on-board; maybe you'll see differently. Those VIA KT266A chipsets are nice, but I'm still leery of VIA after their DMA bug last year that trashed data. SiS opened up something cool with the 735; keep an eye on them.
Go with Athlons for CPUs -- they're 60% the price of P4s, clock for clock, and have a higher instruction dispatch per clock (AMD hired big chunks of the Alpha team after DEC went to Compaq). And more importantly for upgrade paths, AMD keeps the pinouts and electrical standards longer than Intel. Lately it seems like Intel intends for every upgrade to involve trashing both CPU and mainboard.
Buy SCSCI drives, cover them with fans. Almost every hardware failure of mine has been heat problems on drives.
Buy ATI for video if you dislike monkeying around with your kernel. Not that monkeying with kernels is bad (or I would have a big problem), but 'monkeying' usually entails 'distros do not include it'. My debian machine is riddled with little hand-configs and monkeyjobs I have done on it to do things not in the distro -- NVidia is one of those things.
As for a hardware reseller who isn't a pain in the ass, I'm having good luck with newegg.com. I haven't tried anything in terms of support or techs, though. I don't think they do that. For full machines, Talli mentions Penguin Computers. I've only worked with their rackmounts, but those were some very nice machines, for very cheap considering the competition. Might be worth looking at.
As for your last line (new CPUs)..... $300 can buy you a new motherboard and Athlon XP. Then reuse all your cards & such. But junk that 100Mhz fossil.
