I'm going to overkill this machine for what's currently out there, trying to go for the best price/performance ratio, and assume your willing to spend around a grand w/o monitor:
Athlon T-bird 1 GHz - $200 - who can resist saying that have a 1 GHz chip?!
KT133A Motherboard w/ onboard sound - $150 - many threads on this topic, I don't have a personal preference - Abit, Asus, MSI, Iwill
256 MB PC133 RAM from Crucial - $100
ATI Radeon 32 MB DDR - $100
45 GB IBM 75GXP - $150
Netgear FA311TX/modem - $25
Asus 50X CDROM - $50
Intellimouse Optical - $50
Speakers - $25
Total: $850 + monitor - make sure you get a nice monitor, 19" seems to be the standard for performance machines these days
Definitely Windows 2000 Professional, with no substitues.