When I was using a windows pc, it never seemed fast enough. It was a constant race to upgrade one component after another. First a faster CPU, than a better graphics card and than the price of ram drops and I find myself throwing more money into the system. Soon there are plenty of parts left over to build a second system.
My macintosh collection is also starting to grow. As most macs were designed more like a household appliance where end users were generally limited to ram or hard disk upgrades, I start to have a growing collection of older but still usable machines.
Some possibilities suited for an older computer include running an arcade game simulator to relive those retro games like puzzle bubble, lode runner, pac man and the popular daytona. All that is needed is a emulator program and the game roms. The only universal program that runs on both intel and powerpc macs is Mame OS X. Roms can easily be found through a quick search on google. Anything with a G3 processor and above should run the older games easily. Although daytona would require a relatively new macintosh with an intel processor as the only emulator available only runs in windows.
Another option for an older mac is to drop in a bigger hard disk or attach an external drive to run iTunes. That way, it frees up your main computer to get work done more quickly. If you are lucky enough, a 20th Anniversary Macintosh would be the ultimate mac for the job with the integrated BOSE speakers and sub woofer.