Its all a matter of personal preference, but in my opinion 900 is the minimum now. Personally I push it a bit higher now. Even 980 is going to be fine. I personally never go below 900. But again, it all depends on your site's target audience as well.
If you have a site that say sells hi tech toys, chances of the target audience having an old computer set to 800x600 is slim. If you have a site geared more towards the retired age that may have eyesight that is starting to , umm .. well not what it used to be, they may have their resolution at 800x600 still so that the text isn't so small.
So while techically 1024x800 may be the norm, it may not always be the best choice depending on target audience.
Adobe Community Expert, Dreamweaver
Adobe Usergroup Manager, Northern British Columbia Adobe User Group
As others here have already said, identifying your target audience is key to knowing which screen sizes to support. If you know your site visitors are running the gambit from smart phones and other handheld devices all the way up to 3-panel wide displays, now might be a good time to convert your site to a liquid layout (%).