I am surprised by how many people still use 800x600.
I guess you will be using the numbers to optimise the look of your own site, so another statistic you might be interested in is what browser the users have. I recommend testing it against all of these to confirm the layout works.
Also when you are testing, increase and decrease the font size within the browser to see if it affects any of your tables. Often if you set the table to be a size based on a number of pixels, it will be messed up. Best to use percentages to define their dimensions.