What is the best way to center the entire page?

I am working on a site built with Atahualpa 3.5.3 and WP 3.0.1 here:

www.lotuseterneforum.com

When the site width is fixed at say 700px, everything is nicely centered.

Yet when I increased the site width to 1189px, the page loads by default to the left, and img.logo and my new widget area (text6) in the header are no longer centered to the layout.

I would like those to be centered, and I would like the page to load in the center of the visitor's browser by default.

What is the best way to accomplish this? I tried Googling and found this helpful link:

http://davidwalsh.name/horizontally-...-structure-css

I am not sure what CSS ID I need to modify in Atahualpa to apply the "{ width:900px; margin:0 auto; }" suggested, and if this is even the best way.

I tried searching the forum but I did not see a relevant topic.

Thanks for any help!

P.S. Here are the relevant CSS inserts for how the logo and text-6 widget (the ad) are positioned:

img.logo {
position: absolute;
top: 30px;
left:18px;
z-index: 59;
}

div#text-6.widget{
position: absolute;
top: 30px;
right: 18px;
z-index:60;
}