So many new terms come up for dealing with this. You can do liquid design, elastic design, fixed width design etc. I'm not even sure what the difference between liquid and elastic design is!
I still like designing fixed width to 700px or so. A list Apart recently redesigned their site to an even narrower width that that - when many people thought that they would chose an elastic or liquid layout. After using their new site for a while, i have to say that I like it, and the narrower width actually improves readability.
We also use three columns and centre one has space to take up the slack. We also have a few pages where we put in pictures which can be seen at the high resolutions but are covered by the important stuff (using z index) on the 600x 800.