Center div blocks

[wxdqz]

I found a way with javascript to position the blocks to have my 800 pixels content centered in the screen using document.getElementById("imagetop").style.left and document.all.imagetop.style.left, but when the conexion is slow, the menu is centered first and the main block next, even if the line of script for the main block comes first. This doesn't look nice. How can I position the main block first.