If the site you're making is a large and general website (MSN or GOOGLE) then course you'd set your own screen rez down to the specs you're working for. As for tables vs divs, divs get my vote. They are indeed lighter in size and on a large site this could mean the difference of major $$ in bandwidth charges. Overall, I tend to use divs for fixed width sites and some more simple fluid layout sites. When working on large fluid dynamic sites, I stick with tables simply because my skills with fluid layouts in CSS suck ass and I don't usally have the time to learn them before hand til I'm free of any projects.