as for browsers, i also use firebird set to 800x600 (at home and work), but i don't make it taller. i design my sites so that they fit easily on 640x480.
I always check my sites in various resolutions - from 640x480 to 1600x1200, but I don't worry so much about the 640x480 anymore. When I was first doing web design there were only two resolutions to worry about - 640x480 and 800x600. I try to make the main content fit in 640 pixels wide, so if someone has to scroll horizontally in a small resolution they'll only be missing a right-hand side bar.
Of course, I use W3C standards - XHTML, all CSS layout, etc. - so my pages tend to flow pretty well in any web device.
I do, however, often write a handheld stylesheet to make things prettier and easier to handle in the limited 150px-wide window of the smallest Palm screens.
However, this forum is 800x600 minimum - it's horrible and painful to use in 640x480.
And how many people still use 640x480 to surf? Just those at poor companies, or poor people at home. I don't mean to disparage the poor, as I'm using pretty antiquated equipment here too, but anyone can afford a five-year-old computer, and that five-year-old computer will be able to at least display at 800x600. I mean, how much are 15" monitors now? $60?
I use Firebird and have some extensions to re-size the browser to various resolutions for testing, but during my everyday surfing I like to set it at 800x1000, because I can see more on the screen at once. I mean, I have the extra pixel space, I may as well use it.
I'm looking forward to getting a 21" LCD one of these days and moving up to 1200x1024 resolution.