The setup would look a bit like this I suppose:
Mac G5 -> (Ethernet switch) -> ArtNet node -> DMX to ColorWeb panel
Depending on the number of panels you use, you need a number of DMX universes.
I think 1 panel = 48 DMX channels
So maximum 10 panels on 1 universe.
So 30 panels would mean 3 universes
You probably need 1 additional universe to control your G5 from the desk.
And maybe some more universes available for future plans.

Have a look at these nodes to convert the ArtNet to DMX:
Luminex Ethernet-DMX8
If you drop me an email on our Luminex website I can send your further detail about a possible setup.