The one I'm using now is many years old, except for a few upgraded parts, which consist of nothing more than a new video card and sound card, I think. Oh, yeah. A new hard drive or two.
When I got it, it was quite nice. Dual 333MHz Pentium IIs. 256MB of RAM. Of course, now it's nearly obsolete. None of the system parts (motherboard, processors, memory, etc.) can be used in an upgrade, so I've gotta get all new stuff.
So I ordered a new motherboard. It turns out that Intel now makes motherboards that have features and are affordable, unlike before. So, since I'm interested in stability above almost all else, I got an Intel D865PERLL, which has built-in audio, SATA, ethernet, and FireWire, all for about $125, which seems to be a price on a level with other reputable motherboard manufacturers.
And I got new memory. 2GB, which, while the most expensive part of my purchase, was not terribly expensive. And it's from a reputable manufacturer as well, Corsair. One of my friends had big, big, problems with some flaky memory. I became even more convinced that paying for name-brand memory is a very good idea.
I also got a pair of SATA drives. They should be fast on their own — their specs are about the same as the Western Digital Special Edition drives, which everyone seems to rave about — but the motherboard can do RAID0 (I've always had a problem calling this RAID, as it's far from redundant), which should, and is supposed to, speed them up even more, at the expense of a potentially lower MTBF.
I got a new processor, too, but that's not terribly interesting. (It's a 2.6GHz P4.) And a new power supply.
All for about $1100. Not half bad, I say, for stuff I picked out myself, instead of relying on Dell. Actually, a comparably equipped Dell would cost me about $2300. Of course, that would include stuff I didn't get, like a case, software, and a few other things. But I've either already got those or don't want them.
BTW, the online computer parts retailer NewEgg doesn't seem to have the absolute best prices, but they're cheap, and their online store is great. It's not the prettiest, but it's got a lot of information about the products and has an excellent search mechanism to narrow down your choices. And shipping was cheap. $5 for all that, standard delivery. It was only $17 for two-day, but I don't need it that soon. I might not even get to it as soon as it shows up, so it seems silly to waste $12, even if it seems like a good deal. I'd recommend them.