Okay, if not 512 MB, then how about 768 MB? I've got an iBook G4 that's currently running Panther. I don't really want to upgrade the hardware but I've got a family pack so I could upgrade the OS. Everything works fine except I get kernel panics every couple days.
Oh, and my son is actually running Leopard on a core solo mini with 512 MB. It's slow, but it works.