Here's a study of the proportion of monitors using varioius resolutions:
The data for this study are the visitors to the W3C, generally a techie audience. I might be tempted to say the a general audience might be represented by the data from 5 years ago. For that year, the 1024x768 resolution was the most common (54%) and the 800x600 resolution was clearly on the way out (14%).
Here's another study:
Unfortunately, we don't know the sites for this study to know if the audience is more technical or representative of a general audience.
A few articles:
http://www.hobo-web.co.uk/seo-blog/best-screen-size/ which quotes quite a bit from...
http://www.useit.com/alertbox/screen_resolution.html which recommends optimizing for 1024x768 and that was in 2006
Interesting discussion here:
which has a REALLY COOL LINK TO
Of course, what is best for your site depends on your audience. You need to ask yourself if your audience could be expected to be more technical or less technical than average. Do your customers make extensive use of mobile devices? If so, you might want an optimized version of your site for mobile.