Well, let's wait until somone receives one and tells us exactly what chip it is.
If it a new G4, then it may better address DDR RAM. That, and the larger L2 cache, would be worth $500 to me.
However, whether you'll utilize the power is dependent on what you do. Try going to an Apple store, and using their machines with the processor monitor open. Run what you would normally run each day.
If you don't peg out both processors, then you don't need more machine. I always have 20 apps and 25 binaries running on top of the OS, so my processor is usually pegged at all times. I have a full load of RAM, and still get pageouts.
For users who are doing office type work (web, mail, Office, etc.) the Dual 867 should be more than enough. For those working with audio, Photoshop, compilers, or running 24/7 services, the Dual Gig should be fine. If you want to do some rendering, run A LOT of services, are a video junkie, or just need to be the fastest, get the Dual 1.25.
But don't buy anything until we hear some more about what's in the 1.25...