Hi Colin,
My first recommendation would be to build your website using the resolution of 1024x768 - a lot of people use 1024x768 and 800x600 is also still common, so I would recommend that you build the site while viewing in 1024x768 and also try to ensure that the pages are still readable at 800x600.
To give you an idea, these are the resolutions of the people visiting DNN Creative.
1024x768 36.86%
1280x1024 27.75%
1600x1200 5.44%
1280x800 5.34%
1400x1050 5.25%
1152x864 4.86%
1680x1050 3.34%
1920x1200 2.24%
800x600 1.96%
1440x900 1.76%
Bear in mind that the people visiting this site are more likely to have up to date IT equipment, whereas on a site aimed at the general public, you will probably find the lower resolutions are even more popular.
A useful tool for switching between resolution views is a web developer tool for the
firefox browser
Which is available here: http://www.chrispederick.com/work/firefox/webdeveloper/
With this tool you can select which size you want to view the page at and it will re-size your window for you. It also has a lot of other useful tools for testing your web pages.
Thanks,