I notice you always talk about swapping mobos... that's just not feasible with Macs. However, swapping the processor card IS, no need to change boards.
There was an upgrade from the 603s and 604s to G3s and G4s. There might be an upgrade from G3 and G4 to the 970 (but that's highly doubtful, it's probably going to be too different).
Mainboard swaps with Macs is so cost prohibitive, you might as well get a new machine. Remember, they're not like peecees. They're more like specialized workstations: integrated, customized, and wierd.