You should really update your Mac mini to 1 GByte RAM, it makes quite a difference in OSX. On my mini Leopard runs quite good, i like the workflow even on this low-end (for OSX) machine.
It's just a pity that MorphOS won't support all features of the mini, like Wlan, Bluetooth, Firewire (maybe Helios is supporting the mini fw port in the near future?). And the Superdrive is the best argument for finally releasing Burnit 3