Going lowend these days brings new possibilities to the fore. I would respond by observing merely that you can buy a dual G5 tower for not much more than a G4 iMac, and it will be a much more powerful and expandable machine for regular use. If I were to get my hands on a G4 iMac, on the other hand, I think it would just get put on a shelf with all the other old crap.

Currently, I use a 12" G4 PowerBook (1.5GHz) regularly, and a couple of dual G5 towers (home and work) regularly, and can tell you that a G5 tower runs rings around the G4 PowerBook. I suspect that a G4 iMac would be slower than the PowerBook, as the former was a consumer, and the latter a professional product. Cache size, for instance, is likely to be different.

A dual G4 will also bring performance benefits. For instance, I have popped a dual G4 500MHz in a (hacked) G4 Cube that was originally a 450MHz single G4, and the performance benefit was very, very noticeable. In a duallie, Activity Monitor shows both processors working consistently, so that even while using applications that are not designed to use dual processors (MS Office, for instance), there are overall benefits that become immediately apparent. Multi-processor machines are just more satisfying to use.

Now, if a dual G4 could be shoehorned into an iMac, things might be different.... I have never looked into that.