The screen resolutions I use are: 240x320*, 320x240, 1024x768, 768x1024*, 1280x1024*, 1024,1280, 1600x1200, 1200x1600. (* are the ones I use most)
When I design for a PDA I design for 240x320 but take care to make sure the site takes advantage of landscape as well.
I design most sites for 1024x768, however I often allow for a reasonable sized favourites bar to be displayed in the browser as well. This comes out to be just a bit bigger than designing for 768x1024 (no favourites bar). Where feasible I will usually make part(s) of the page stretchable to accommodate widths of 768 to 1024.
I rarely design for 1280x1024. If I do, the right part of the screen is just a bar with some crap on it, internal links, etc. So many people never see it that it's not worth spending time on.