simple layout question

i'm trying to build a page that will centralise in both 1024 x 768 & 800 x 600 browser windows. so the page will fill almost the full screen of the 800 browser & have right & left borders in the 1024 border. the site http://www.redrhino.co.uk is an example of what i'm trying to achieve.

my efforts so far are just producing a page that looks ok in the 800 browser but appears on the left of the 1024 browser with a large vacant space to the right.

any help would be very much appreciated.

thanks