If at all possible, go take a look at these things or at least ones of the same size.
I go the 24" screen and it is H U G E !! Actual physical dimensions on it are 22.5" wide by 20.5" tall. That's the entire thing, not just the screen.
Make sure you have enough room to put this beast on your desk. I know one disappointed person who took their 24" back because it did not fit in the furniture!
The 20" screen is a nice size and has a good picture, then again, so does the 24"
As your question was primarily about screen size, my answer is Yes, I think the 4" difference is worth the extra $100. Of course, that's just my opinion.
In my last couple posts I have decided to buy a Mac, decided when to buy a Mac, and chosen the iMac instead of the Macbook. Now for the grand finale of decisions:
$1,299.00(+Tax):Refurbished iMac 20-inch 2.66GHz Intel Core 2 Duo
20-inch glossy widescreen display
320GB hard drive
8x SuperDrive (DVD