Here is someone who did some pretty intensive hashing of info on the subject - however, their sample was a bit small (under 1k)
http://evolt.org/article/Real_World_...at_II/20/2297/
The number of 800 X 600 browsers has been overtaken by the number large, the whole thing has tipped over (probably last two years) to bigger sizes. As people upgrade pcs and move to newer systems, I think you will see this number move up.
Also, LCD screens tend to be a bit sharper than tubes, and most of them have a native bet mode around 1200 X 800 or so. My LCD 17 inch monitor can display upwards to 2500 X 1600 very sharp, but I can't read it!
Just when you thought you had enough of screen resolution issues, remember that your TV is being revamped too! It isn't getting better, it's getting worse!
Alex