I would certainly NOT wait based on the new operating system. The first release of Leopard is going to be riddled with bugs and incompatibilities, as they always are. So, your true ability to use Leopard is not going to occur until months after the initial release. I try never to buy the first version of a new OS or the first iteration of a hardware release. For example, there's many obvious ways the Apple TV can be improved, and I have no doubt you'll soon see an Apple TV that can show your iMovies, etc - so I'm waiting on that. But with the iMac, you've got a stable product whose only real changes in years have been monitor size and the switch to intel. I have an iMac G5, about 2-1/2 years old, and it works like a charm. I don't anticipate even wanting a new machine for at least another year or two, and even then if they manage to come up with a way to swap in an intel processor, I'd do that. (Currently there's none.)
Dave