2
i prefer fixed width themes myself.. i think they allow for a more professional looking site.
i usually optimize for 1024x768 displays tho. I find the vast majority of people using 1024x768 exceeds the 800x600 displays these days..
also i think 1024x768 is a minimum for 3 column sites.. 3 columns on 800x600 is a little cramped and not professional looking at all. i would stick to 2 columns if i had to optimize for 800x600 resolutions..
allowing for the scrollbars, i usually set widths of around 990px to be safe.