Not sure I'd term it "upgrade"...I would just buy a new motherboard/CPU, PSU, RAM and case.
PII systems are quite obsolete compared to the systems/technology which have emerged since then.
If an "upgrade" was doable, I doubt if it would be worth the effort...considering the prices of any number of components which are better quality while not commanding "brand new" prices today.
Example: One Vendor From www.pricewatch.com
Seems foolish to me to even think about spending money on such an obsolete system when the money could be better spent on something more current, yet inexpensive.
Even better, IMO, is a system that includes an O/S and proper license...such as http://www.pricewatch.com/gallery/computer.../athlon_le-1620