Check the worldwide browser market share statistics
This questionThis question provides a comprehensive list of sources to look up worldwide browser market share.
Here are the top 3 2010 averages for StatOwl:
1024x768 26.75%
1280x800 18.69%
1280x1024 11.57%
Here are the top 3 2010 averages for Net Applications:
1024x768 24.27%
1280x800 17.98%
1280x1024 10.83%
According to the data, your resolution minimum should be 1024x768 but it also depends on your demographic.
If your site is geared toward a more corporate audience stick to 800x600. A lot of corporate networks are still running really old hardware with Windows 2000. If that's the user base you're targeting, using the 'current norm' of screen resolutions may alienate your audience.