That's tricky because site visitors use screens with different resolutions. I usually try to make the site readable and usable by everyone, so I go for 800 width. For mini sites, I go for 600.
I use only 980. I don't know why but I think it's enough for all kind of websites. I don't bother to search for a perfect width, because 980 is the best, for all websites.
Nowadays 1024 screen width is quite common - 960 width is idea for me. I will keep it to a minimum so a visitor doesn’t have to keep scrolling left and right.