Have you figured out how to remove the current hard drive? Some are easy and others...not so easy.
Depending on what you plan to do with it, it might be worthwhile. It won't take much RAM and with an IDE/PATA 2.5" drive, it won't do anything quickly. A new battery would be a deal breaker, so if you do anything with it, it will probably be semi-stationary and used for checking email or posting on COAR15. If you do it, check out some of the lighter Linux distributions like Puppy or Linux Lite.