i was faced with the challenge of recently deciding on a new imac too.
I ended up going with the G5, only because I couldn't wait a few MONTHS to get creative again.
I can honestly say that this machine takes everything I can throw at it, for now at least.
If you can wait...wait. It'll be worth it in the long run. Just remember that some top notch artists make top notch tracks on not-so-top notch gear.
My friend Jon Pegnato has a stack of releases out on many labels, inlcuding his own, and his tracks have been featured on DJ Dan's latest mix compilation. He's on a Powermac G4 still.
It aint about what you have, its how you use it. Good luck man.