and welcome to the Forum
Windows 2000 is hard to find and when you can it is expensive. You may find one on eBay. XP will likely have all the drivers for the devices that came on the Optiplex. . any that it does not can be downloaded from the Dell Support site.
The upgrade from win2k is pretty straightforward and easy. Just put rhe XP cd in and select Upgrade . . .
You can get a reinstallation guide specific to your Dell here
http://support.dell.com/support/topi...n&s=dhs&~ck=mn
This is a great guide for reinstalling on a Dell:
http://www.djdenham.com/Install%20Procedures.htm