Like everyone said, it depends on your budget and what it's used for.
I'll take it this will just be a plain webserver box (no dns, no mail, no ftp, etc, just http/www serving). For static pages get at least a P500 or a Celeron 500 and minimum of 256mb of RAM (OS dependent, if you have Win2k the more the better).
Depending on the traffic you're expecting you can stick with IDE drives, 7200rpm with 2mb buffer is preferable. For higher demands go for SCSI/RAID or IDE/RAID. For extremely high demands you can also get a caching RAID controller (64mb minimum).
A good NIC is a must. However i noticed, after 2 years of hosting those Netgear and Linksys NIC's work just fine.
For your reference, the first ever server i used (and is working flawlessly up to now) is the following:
HP Pavilion (Yep, the home computer) Celeron 500
64 MB RAM (since i bought it i've already upgraded to 384mb)
15 GB 5400 rpm drive
Netgear NIC
I got the whole system refurb for $300 a couple of years ago.
Believe it or not, this baby handles approximately 500mb of traffic (on average) a day and handles FTP/DNS/Mail/WWW for over 130 domains.
Never slowed down a bit. 🙂