I recently bought the Dell 24" monitor for my PowerBook, which works great. However, I had a hard time getting it to work properly with my girlfriend's G3 iBook (933Mhz). The there is not enough video memory to drive both properly!
The only way to get a decent screen resolution is to install the screen spanning software, and then switch the iBook to display in 640x480 and at 256 colours. This will then enable the iBook to drive the Dell screen at 1920x1200 using millions of colours and have a steady picture.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060322091636939