The answer depends on who your target audience is and how important it is for you that they be able to see all the content. 

According to the most recent stats that I've seen some 20% of the population still uses the 800x600 resolution. In my experience, these are often older users and people who are not necessarily computer-savvy.

If the last sentence describes your target audience (or else your website is general interest and you want to make sure that your users see everything on their screens) stick with 800x600. Else, feel free to go for 1024x768.