I wouldn't go for Win 2K unless you have to. You can change the look and feel of XP to be the same as 98 / NT if you want to, via a simple Control Panel > Taskbar and Start Menu and select "Classic start menu"
XP is easier to configure and maintain and I'd only recommend 2K for a networked business environment myself.
EA
PS XP runs fine on an Amd Athlon 600MHz, however unless you have 256MB (preferably 512MB) I wouldn't bother running either 2K or XP, I'd stick with 98