It depends on what your ultimate goals are. If you want to run OS4.x get a real PPC motherboard. I have the Bliz PPC 060 and while it's ok, the PPC is still slow enough to make you wished you had spent the money on a newer motherboard with which to run stuff.
If you do WHDLoad games often, you might as well stick to what you have and get a PPC motherboard to run OS4 or MorphOS on. If MorphOS is ok, just get a MacMini G4. You can get them for like $100.
