Donating older computers (within reason) is good for everyone. There’s no call for anything pre-Pentium, except for possibly a museum, but Goodwill and the like can put computers ten years old and newer to good use. Make sure you wipe your files, though. A good tool for that is Disk Wipe (http://www.diskwipe.org/). It’s 100% free, and will securely erase your sensitive data.