There's a lot in my stats about the screen resolution my visitors have, but I never run my browser maximized. ( With FireFox you can even deny javascript the right to resize your browser. ) So my screen size doesn't say that much about how many pixels can be devoted to a web page.

On the other hand, I've seen a lot of fixed width, centered sites lately, usually in the ballpark of 750 pixels wide. I probably give FireFox about 1,000 across, so that leaves a lot of wasted space, and just doesn't look very good. You can avoid that with a fluid layout, but then there are questions for images, flash objects, and whatnot.

So I'm curious what other webmasters use, and what they design for?

I put it on 800 wide for content.
My layout goes wider for people with wider screens, but the content is centered so everyone can see it.