For you I recommend two things:
1. Buy the iMac.
2. Pay attention to the apps you buy: buy Universal Binaries.
If you needed some PPC-only (non-Universal apps) right now, then I'd recommend the G5 tower. It's a great machine too and will run most apps for years to come.
But otherwise, save some cash, save some space, save some noise, and get the FUTURE (Intel) processor line instead of the outgoing line. Plus, as a multi-platform user, maybe someday you'll like being able to run Windows and your choice of x86 Linux on your Mac, and keep your old Windows apps without having to keep 2 machines around. (You can't boot Windows on an Intel Mac yet, but people are working on it, and Vista should run when it's completed.)
I'm thinking since you can hang onto your current PC systems as well, that you need not be in a rush for any certain apps. Therefore if a certain app is not Universal yet, you can just wait for it.
Also, even though most apps will stay PPC-compatible for a very long time, the first apps to go Intel-only are likely to be games. There was an interview with Mac game companies at
http://insidemacgames.com and some companies said they'd start making some Intel-only games in a year, some in three.
Now, many current Mac games will NOT run well on Intel. BUT patches to make them Universal should be hitting the 'net in the coming weeks. Problem solved.
The main reason NOT to get the iMac is that you can't upgrade the GPU. But beyond that it's a great buy.
If you want the best of BOTH worlds, you could wait until fall or summer and get an Intel-based PowerMac. Those should REALLY fly (Conroe-based). But they'll still be big and ungainly compared to that iMac
