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The size of the page is slightly influenced by the tags... A table requires opening and closing table, tbody, tr, td tags where CSS uses an opening and closing div tags. However, this is a few bytes difference so is negligible. The real win is that the pages will render slightly quickly with <div>s as the browser needs to parse the whole table to work out how to render it.
Check out the CSS Forum [webmasterworld.com] for tips and techniques on how to achieve layouts using CSS rather than tables.
HTH
We went from a tryout of all-div design (about a year ago) to using a hybrid approach today. We use a container table for the layout's "frame" and then we use divs within that frame. This gives us faster production times and much more certain cross-browser compatibility.
The results doen't seem to lack anything that I can see, and the clients report strong results on their bottom line metrics.