Lspence is correct, fluid design.

Although... That works great for the general layout of the website. But the images will not be fluid. So your images will be smaller on the larger monitors and larger on the smaller monitors.

Some other suggestions to help:

1). If you plan on a significant amount of traffic coming from handhelds (pdas/phones), I suggest building a separate design specifically geared towards the pda's/phones.

2). For width/font sizes use px and not pt. Px fonts will resize better depending on the size of the monitor. Pts should really only be used for CSS print.

3). What I try to do is: Find the most common aspect ratio (which I believe is 1024 x 76 and build for that. And explain to your customers that you build websites for what the most people are using. I usually: build a max of 955 wide and center that or left justify.

There is no 100% way to determine the most popular resolutions, and W3C can't be 100% because it bases its stats on its own visitors. However, I believe its the best we got at the time.
Browser Display Statistics