800x600 on a 19 inch flatscreen. Maybe it's my old video card but at even the first setting higher than that everything is too tiny.
I design my pages for 800x600 but do occasionally take a peek at 1024x768 just to make sure nothing is out of whack.
Another thing you should remember to do when checking out your pages at different resolutions is also check out different text size settings too. I've seen many pages where the browser text size setting makes a white space separation between all the sections of sliced images. An example of a page that does this to me is Juicy's webmaster page:
