An iMac G5 sounds better, in my opinion, compared to the Mac Mini. Of course, the Mac Mini is cheaper and allows you to move to an Intel Mac with lower spending on your part, but the iMac is based on G5, which is a better and much faster choice in today's world.
For a Mac Mini to perform well, you'd have to add a lot more RAM - a reasonably well built Mac Mini (again, my opinion), costs around $1000+. And it's still a G4!
Since you've been happy with your Cube, the Mini might probably be a better fit for your budget. But going for the iMac G5 is not a bad choice either (it is future proof for many more years - the basic configuration comes with a very good monitor, wireless, bluetooth etc.).
I don't recommend that you wait for the Intel Macs - they're still more than a year away, and with this kind of an architecture transition, you wouldn't want to have a Rev A model. An additional factor to consider is that most of your apps today would be running on emulation on the Intel Macs for a couple of years (even if native Intel versions are available, you'd have to be willing to probably pay and upgrade).
In my opinion, for a normal consumer who has a Mac right now or is considering one for now, going for an Intel Mac in 2009 sounds like a pretty good time frame (all software and hardware issues ironed out, with more native support for the Intel platform).
