One of the old studio LCDs would also be a great option. I own one and adore it. Mine's only 15", though, but I got it PLUS the ADC-DVI convertor (retails for 100) for like 200.
Here is a Samsung Refurb that I found for under $500, 20inch widescreen. We have three Samsung monitors here, two CRT's and one LCD, very very nice display.
i know that if you have an ibook it will only mirror its own resolution, but i have a powerbook g4 12"... from what i understand it automatically detects the display and puts the display in its native resolution, correct me if im wrong