Here is an interesting page with some stats on screen resolutions, operating systems and browsers.

Good link - thanks for that.

These statistics come from W3Schools, so they are not the perfect indication. But their good to get a general idea about trends.
From their site:

W3Schools is a website for people with an interest for web technologies. These people are more interested in using alternative browsers than the average user. The average user tends to use Internet Explorer, since it comes preinstalled with Windows. Most do not seek out other browsers.

These facts indicate that the browser figures below are not 100% realistic. Other web sites have statistics showing that Internet Explorer is used by at least 80% of the users.

Anyway, our data, collected over a five year period, clearly shows the long and medium-term trends.

I looked several others. Most are about the same

I just checked my stats and see that less than 10% of the visitors are using 800x600 or smaller.  That's still enough, though, for me to want to change the layout of the site to be more accomodating.

The smallest resolution?  320x240
The largest resolution? 3840x1024

My stats are a little different, but consistent between 2 of my sites:

1024 x 768    51%
800 x 600     22%

I'll keep building for 800x600 til the numbers justify otherwise.  I often wonder how many folks out there don't even realize they can change their resolution.