Title sounds easy right? Just check out W3Schools and you can see that about 95% of web users use 1024x768 or above. Soooo, lets crack open photoshop or flash and create a canvas 1024 by 768. Right?
That would be fine and dandy, however, you are forgetting about the browser. You are forgetting that most people have Navigation Toolbar, BookMark Toolbar, AddressBar, Tabs, ScrollBars, etc... You drop your lovely 1024x768 into a browser and suddenly you are clipping 100 pixels off the top and another 20 off the sides.
Solution? Some of the creative folks here at the basement (Brian and Amy) use a browser.psd template when they start designing a project.
It is just a png of the browser that they lays on top of their design. Many times they will have their design expand out beyond the browser's frame so they know what the site will look like when it scales... but they keep the frame here to turn on whenever they want to see the site at its minimal size. This way, when the site goes live, no one is surprised by the extra space taken up by the browser.
Hopefully you find this helpful.