Frances, it will be a bit slow to do a backup via the network (I'm assuming wireless - if it's wired it won't be so bad) so if you can connect your Time Capsule or your external Maxtor via Firewire or USB 2.0 just for this exercise, it will probably go a lot faster. I'd suggest setting it going when you're not planning on using the iBook for a few hours too, as the first backup takes a while. Updates to a backup don't take long at all, but the initial backup can take hours if you've got a lot of data.
Then you simply set up CCC to clone your drive to the target external drive (copy everything) Do you have anything on either of those drives already? If so, you should move it to the iMac or the other drive. Cloning drives to folders is possible, but makes it difficult to restore, so is not recommended from what I can see.
You should have no problem using Leopard on the G4 Powerbook. You can always feed it some more RAM at some point - it has 512 soldered to the motherboard, and 1 slot, so you could take out the 512MB in the slot and put in a 1GB stick. But see how it runs with 1GB. If you're planning on using Photoshop, do be prepared for it to be considerably slower than the iMac, but if you get the screen sharing going, Photoshop will be running on the iMac anyway, so no worries.