well i did ask about resolution recently but im specifically asking about designing for above the fold
Except for this bit: "do you take advantage of the extra width or make sure it stays in 800 x 600 res?"

As far as being above the fold is concerned - how high is that? Even if everyone was on the same resolution, you can't reliably predict where the fold would be because you don't know which browser is in use, which toolbars are visible, whether the browser is maximised or not; so it's still a good idea not to have too much 'furniture' at the top as it can still lead to the need to scroll too much before the actual content is reached IMO.