Well,
If you want you web site to be the Full Screen (no metter what resolution a person has) your web site will always be the full screen then you need to make your tables in Percents.
If you want your web site to be in the middle then you need to center it and use pixels. Pixels are better because your web site will always have the same size. (I had problems with that. My resolution is 1024*768 and the page look good on mine, but when the person whos resolution is for example 1280*1024 visits the site he/she sees not what you want. The tables start fly (i call it fly, maybe you call it differently).
To see what I mean you can look at this page:
http://www.creationage.com/elegance/ (This one will have the same site with every resolution, I mean like I want it.)
If you look at this page:
http://www.creationage.com/elegance/contacts.php
try different resolutions you'll see what's happening to the tables.
I guess I could help you. I could explain it with mistakes, but I think you'll understand me. (I'm russian and It's hard for me to explain it in english).