I use
Computer Sonics for almost all of my computer stuff. They're just south of SouthCenter. They have some good deals on "bare bones" upgrades -- case, power supply, motherboard, memory, CPU. Move over your cards and hard drive, and you're set. AMD 1GHz with 512Mb RAM would run about 300 + tax.
Although, If I was taking such a big step (from a P5-166 & W95), I'd take a close look at your hard drive size, and the cards inside that you intend to keep. For a little more money, you could get one of their complete PCs -- Hard drive, killer video, fast CD drive, and Windows 98/XP pre-installed. AMD 1GHz with 512Mb RAM, 20Gb drive runs about $500. They'll do custom builds, if there's something else you want/don't want on the box.
Oh, and if I can run my mouth a little more, upgrade your monitor, too. A decent 19" is about $200 right now -- and that's the part of the computer you use the most. If you need help, drop me a note, I can lend a hand.
-jon-