Eerrr.... What about a cheap and good Microcontroller ? MPC5200 or AMCC440EPx...
Have you seen MPC5121e ? Looks really nice :-D
I would not go for a G5 monster attached in an old Amiga, but a 20 bucks microcontroller at 760MIPS would give me a lot of fun.
Going for a bigger PPC would be nice to a brand new machine, in a tower, etc...
May I dream, at least ?