The G5 is the PPC 970, which is based on POWER4. The G6, which would be the PPC 980, would be based on POWER5. POWER5 is just now in test stage, meaning that the design is almost completely done. Since IBM and Apple by now have already the experience of adapting POWER4 to PPC970, it should be easier to do it a second time. I do not think it will take long, maybe two years or so, before they could be ready. However, the G5 can quite easily reach 3-4 GHz on its own.
BTW. the G7 would then be based on the POWER6, which is currently under development. With this chip they could immediately take into account the possibility of adapting it to PPC 990 already now in the design stage. Who knows what the future will bring ...