All sizes are used, and it is possible to design the page to work with all of them. At work I have one size no one has listed yet, at home something lower, so with Buz, SoCalHiker, the others and me we are up 3-4 sizes already.
A while back someone posted this website. I have found it helpful in checking that a page looks ok with all sizes.
http://www.screen-resolution.com/#
Also I keep photos to 740px wide or less, so it shows ok even if someone's resolution is as low as 800x600.
I'd recommend folks put this little bit of code after sections of a page with large or several photos on the right, but not a lot of text. Very useful in dealing with multiple screen sizes.
<BR CLEAR=ALL>
This keeps the photos in the correct section, so a page looks like this:
instead of this:
EDIT: I removed my screen size because it shouldn't matter.