I say you should still keep those 800x600's in mind.

With a bit of work, liquid layouts can aid all users - those with bigger screens can enjoy the extra space they have, whilst smaller screens don't have to suffer the bane of horizontal scrollbars.

Unless your layout or design is extremely inflexible, there's really no reason to code otherwise.

You don't have to design sites that fill the entire screen area, which can end up looking stretched and distorted. I tend to use min and max widths, serving a max of around 1000px and a min of 750px.