Just got my new MBP. Im lovin it, but my powerbook G4 has nothing to due. What have you guys been doing with your older systems when you upgrade? Im thinking of maxing out ram and using it in office/study....
You could use it as a file server... I found it useful having two machines because it was such a good backup. If one machine went down I'd keep all my files and settings identical on the other machine and lose the minimum amount of downtime possible.
all these are good ideas. HDD storage is a nice option if you want to save pictures/videos/music, etc.
Or, selling it is also a good idea...
Otherwise, giving it away to a potential switcher could maybe even be the best idea. You could potentially save them from many frustrating years by giving someone their first Mac.