What I have now: iBook G4 12.1" 1.33GHz, last revision just like you. I do basically the same tasks as you, word processing, IMing, internet + email, some photo editing, nothing too processor intensive like video editing.
What: iMac Intel Core Duo 17"
When: Well, strangely similar to you, I'm probably going to grab one a few months after Spring '07, after Leopard will be on there and I'm sure a couple revisions here and there will be made to maybe processor speed or hard drive by that time.
Why: Don't get me wrong, the iBook is a great machine, but I've really been longing to get a Mac desktop for quite some time now. I got the iBook just about a year ago for school, and I'm sure it will last me until I graduate. I'm only in my freshman high school year now and this baby seems to have a lot, and I mean a lot of life left on it. I'm mainly getting the desktop as.. well.. a desktop for home. I currently use an HP laptop as my desktop replacement but I'm longing to get rid of that and Boot Camp Windows up on the Intel iMac once it's in my hot, little hands. Plus, using some more processor intensive programs like iMovie and working faster in GarageBand wouldn't be so bad either.
Waiting is damn hard though... I wish Apple would release Leopard and whatnot now.