Here's a good source of stats on browser resolution. I think its good policy to bear in mind the 800 x 600 users when designing a site. According to PCPitstop it about 35% still. http://pcpitstop.com/research/vidres.asp Let's just be greatful the 640 by 480 has almost completely dissappeared.
Some other stats I've seen put the percentage using 800 x 600 at closer to 45%, although it has definitely been dropping in the past couple of years. Whether that trend will continue or reverse itself as internet users age and get into the reading glasses stage of life remains to be see. From a designers point of view, I still recommend 800 x 600. This means that visitors using that resolution can view your pages without sidescrolling and visitors at higher resolutions can view them comfortably in a non-maximized window.
I have dual head monitors with one set at a lower resolution so I can preview what a page looks like at different resolution. It's easy to forget that every one see things differently especially when they are browsing on a mac using IE 5.1 - that browser is evil and stole many hours of my life.
I use bookmarklets to resize the browser window to different resolutions. That way i don't need to change the resolution when testing. Here's an example: javascript:void(window.resizeTo(800,600)); You can just save that as a bookmark in Firefox, Safari or any other browser that supports javascript in bookmarks.
If you want to check your site in 800x600 (and I strongly recommend you do), the web developer's toolbar is available from http://chrispederick.com/work/firefox/webdeveloper/. It's pretty useful - it'll resize your browser to any resolution you choose, and has also got loads of features such as stripping CSS (to see how your site looks to the all important crawlers) and validating your (X)HTML and CSS with the W3C (it would be nice if all websites did that...)