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Dwarf Fortress is a deceptive game- it looks like a primitive ASCII game, but it places extreme performance demands on even modern computers because of the enormous volume of pathfinding and item handling that the game has to do (and Toady has admitted that the code is underoptimized.) World generation is also highly resource intensive, although you only have to do it once.
I'd be very surprised if you can get acceptable performance out of even an older version of Dwarf Fortress on a PowerPC machine. .40d was almost unplayable after a couple of migrant waves on my 1 GHZ G4 Powerbook, even on 2x2 embarks. World generation could take an hour or more depending on parameters.
As far as I can tell the major limiting factor is that the front side bus on the PowerPC 74** series is horribly slow- it was designed for single-channel SDRAM and can't even run DDR1 at full bandwidth. Dwarf Fortress performance is highly tied to RAM performance.
The last generation of G4 machines were very sad- packed with high-performance components that the processor was completely incapable of using to their full capacity. They put DDR2 in the last generation of Powerbooks even though the machine could only run it at something like 1/4 of its rated speed.