How important is it to make a website layout fit on 640 x 480 and 800 x 600 displays? For some time I have been designing with the assumption of at least 1024 x 768, but I haven't been doing it professionally (just on my site which is just a blog that only 10-15 friends read). Does anyone have any non-anecdotal statistics about the distribution of screen resolutions from real web users?
Note 1: "Non-anecdotal" means please don't give answers like "I know lots of people using 640x480" or "my mom runs in 800x600 so it must be really common." I'd rather have actual data (or links to actual data), especially if it is data about general users (rather than geeks).
Note 2: I'm not concerned with extremely-small displays like those on PDAs and cell phones (at least not at the moment). I'm just talking about desktop/laptop monitors.