i say go for it! the difference between the ibooks and the titaniums is the drive, cpu, screen, and price. the big screen is nice, but i dont think its as easy to see as the ibook, so im neutral in that category. plus it makes the G4 have a bigger footprint. if you dont care about the drive, and you can get a good deal (and thats a great deal) on the G4, i think long term you will really appreciate it. i have a G4/400 tower that i bought close to two years ago, and with OS X and all the new apps that are coming out, the G4 is just going to last you longer. apple has really been using their G3 chip out; its just not the chip for all the new applications being made. for current apps, sure the laptops are comparable. the ibook might even be a little quicker because of more mhz. but that wont be the case for long. bottom line - the ibook gives you better bang for buck, but the G4 will be a great machine for years to come. and if youre getting them for the same price, well...
i would just be very suspicious of a titanium for that price. make sure youre not getting a lemon machine.