This kind of question could probably be answered by someone with access to a single chip G5 and a portable........without the need for a mini.
if you were to kit out a G5 imac (1.6 closet version to mini) and a macmini 1.4 (macmini - you could even say an emac or powerbook really) with the same memory and work with an external firewire drive.......
- the processor mhz on the G5 isn't that much greater
- the bus speed on the imac is about 3 times quicker
Would you expect to just be able to run a few more effects on the imac or would it leave the mini or powerbook for dead?
(leaving for dead would be the mini with CPU usage near 100% and the G5 at 10-20%).
If the imac is say 3 times the price, but has a potential 5 times higher than the mini, then even though, I still have a monitor, keyboard and mouse, the imac is still worth considering.