So I'm checking out the prices on Dell notebooks compared to Powerbooks -- don't look at me like that you zealots! I'm just shopping around a little!

-- and Dell offers three types of displays on their widescreen 15" model:
15.4" Wide Aspect XGA Display
15.4" Wide Aspect Super XGA+ Display
15.4" Wide Aspect Ultra XGA Display
Can someone tell me what the differences are in XGA vs Super XGA+ vs Ultra XGA? And what type of display does Apple use in the PowerBooks?
Don't worry, it's still 99% sure I'm getting a Mac, but it doesn't hurt to see what else is out there.
Thanks,
Mike