Just to provide an alternate viewpoint...
Since you have the latest iBook G4 rev. (you do, right?) and you've put some RAM into it, you'll get a decent resale value, which will probably be about (if not a little more) than what you'd have to pay for an iMac. With one computer (the MacBook Pro in this case), you dont have to worry about syncing your data between two computers or anything like that, and you'll always have the power with you. Personally, I think you should sell your iBook G4 and go with the MacBook Pro. If doing so makes you nervous, give yourself the option to sell it (start a "feeler" thread in the Marketplace or post on eBay with a very high reserve), and if you would end up getting more than the cost of an Intel iMac, then go for the MacBook Pro. Offer peripherals that you won't need if you have them, that'll obviously increase the resale value of the iBook.
Also, be sure to take your needs into consideration. Do you need a big HDD and a dual-layer SuperDrive? If so, go with the iMac. If not, do what I suggested above.