A good idea is to design website to look good with 800x600 and 1280x1024 size screens at the same time.
Put your most important stuff in 800x600 space and less important around it or to the right side of it.

I don't design for anything under 1024 width anymore. People with smaller monitors will have to scroll horizontally, sure, but they'll still be able to see everything; and I don't want to compromise the experience of the vast majority with larger monitors by trying to cater to the lowest common denominator.

This isn't to say I try to use every pixel of screen space--far from it. I find that 900-950 pixels in width is just about perfect, even for 1024x768 screen sizes. If you get too wide, you run up against the fact that humans generally have a hard time reading things with large line lengths--thus, you inadvertently decrease the usability of your site. Also, proper usage of whitespace is key to keeping your site user-friendly--however, there aren't many guidelines as to where or how much is the "right" amount of whitespace. I personally prefer something like 12 pixels of padding between text and any other elements, but I've seen people use no padding all, and others use 50 pixels. It's mostly personal preference.

I don't really design anything for 800x600 anymore either. If it all fits right with the body's min-width at 750px or so, great. If it doesn't, I'm not too worried about it. If it doesn't fit in 1024x768 and is too stretched out at 1920x1200 (which is what I design everything at) then I make some changes.

Google Analytics shows that roughly 1.7% of my visitors (not a tech-oriented site) are on 800x600. At least 97% (probably 98%) of my visitors are on a resolution of 1024px or more of width. As such, I'm happy to design at 960px; those on 800x600 can still scroll over, but there aren't enough of them to justify a degraded experience for everyone else.

Incidentally, the massive widths of modern monitors is rendering the "fluid versus fixed" discussion obsolete. Who wants a page fluidly expanding to 1920 pixels wide?

Well, not entirely, fluid still has applications, you just have to start thinking more seriously about max-width. :-)

The rise of small monitor hand held devices are changing this argument considerably, my sites still fly pretty well at 600 pixel width.

I shoot for an "expected" average of around 900 but with min-width of 600, max-width of 1050.

Not me. I limit my sites to 1280px width just for readability and it gives me better control on the layout.

Optimal screen sizes are difficult to find these days. Small as a mobile phone, large as wall spanning plasma screens. I don't apply a "site width" but I do apply div/column widths that degrade gently for small screens and remains readable on wide screens. We have such a large moving target of display options it is dang difficult to say "this is standard, code for that".

And of course on larger screens people don't necessarily have the browser maximised anyway.

USPS_com home page is an entertaining example which minimum screen size is still important.

Make sure you see it on all screen sizes.

@piatkow - Exactly. My browser at home is never maximised. I actually get upset with sites when I have to stretch the window out to fit their design. A higher resolution screen is not necessarily a sign that people are using all of the available real estate.

For reasons I fail to remember now... when I last redid my sites, I went with 740 centered in the page. I just like it for some reason. I actually don't really think about the tech parts that much, but I just think of it in terms of a traditional publishing page size. I don't really like things that get too wide. But I noticed the other night that a very large web hosting company redid their site, and it's almost the same exact dimensions as mine. So I felt better after that.