Well, I think you'd be pretty damn impressed with a dual-1.0GHz. Even if Apple DOES release the much-speculated-about G5 or some faster G4s, big deal -- it's not like they've perfected nanoprocessing direct silicon 6GHz machines or anything.
Look at it this way: you get a bump to PC133 RAM, and your processors will run TWICE as fast as your old ones -- throw in the dual-processing factor, and I think you'll be perfectly happy with a dual-1GHz machine. You also get a better video card. It's an upgrade all the way around, and a damn good one in my opinion.
Even if you DO wait for faster G4 processors or the G5 processor, what do you think will happen then? 6 months later there'll be faster stuff released anyway.
I think people are misled into believing that when they buy top of the line it's gonna stay top of the line for a few months. Not so, at least in the computer market. The technology is growing exponentially, and newer, faster stuff is coming out daily.
Look at me -- I bought a G4 almost 2 years ago, and now it's obsolete and considered slow. But it works great for me.
Go for it.