It depends really. For apps that only take advantage of one processor (games, for example), the G5 would be a good deal faster. But for well optimized processes, the G4 could easily hold its own. For just general web browsing and such, the G5 would probably be faster in terms of speed, but the G4 would be more fluid in multitasking apps since it can offload to the 2nd processor. Still, I would think the G5 would be faster overall. Remember - 2 x 1GHz does not = 2GHz.