One of the problems you have with sites that expand to fill the width of the screen (such as this board) is that line lengths get too long to be easily readable. Even if you have a wider web page, you need to constrain the width of text.
Newspapers and magazines recognize this. It's extremely difficult to read an 8.5x11 page that runs the full width of the page. Can you imagine what it would be like if the New York Times were printed in a single column? A lot of websites have 468-pixel width for text, but even this is too wide for comfortable reading.
Get yourself a paperback book, and type in a few paragraphs. You will want to adjust type face, type size, and the width in which the type is displayed until you get about the same line length. Too narrow and too wide will both make the site far less readable.
People with higher-resolution monitors don't usually surf full-screen unless they have T-1 connections. If you have to 30 seconds for a page to arrive, you have something else open, so you can do something other than twiddle your thumbs while waiting.
In the near future, we can expect 83% annual growth for internet appliances, and only 9% for regular computers. The most common width for internet appliances is about 560 pixels. People who browse with
a 640-wide 1200-tall browser at the left side of their 1600x1200 monitor find that width to be quite acceptable as well.
If your site makes me resize my browser constantly - narrower as soon as I click, then wider when the
page arrives - I'm likely to think your site not worth the bother and I'll google for a friendlier site that has what I'm looking for. Obviously, there are exceptions, and obviously different people behave differently.