Well, not really a PHP question, but a design/layout question, eh? I think the answer is based on if you still design for users with browsers set at 800 x 600 pixels, or if you design for users with 1024 x 768 pixels and up only. Also, are you planning a fluid design, a liquid design, or a static layout?
A lot of people design with a Max width of 760px in a rigid layout with a centered div so as not to alienate people with 800x600 settings or people who do not fully maximize their windows....
But I can "blah, blah, blah... " on the subject. Why not pose the question on the sitepoint CSS/layout forum: http://www.sitepoint.com/forums/forumdisplay.php?f=53
