Ok to be honest you are not really going to be able to keep anything from that old PC. If you get a new motherboard you will need a new CPU and new RAM and a new power supply and a new hard drive and you see where I am going with this. All the standards have changed since that Athlon was new and about the only way to find old parts is ebay and I would not buy used computer parts myself.
The case may be reusable but that would be about it. Trying to reuse those old parts in a modern machine would be like buying a Ferrari and installing some VW Bug parts just becasue they are laying around. Bad idea.
If you will follow the guidelines listed here : http://www.tomshardware.com/forum/261222-31-build-advice
Then we can help you better.