I had a quick look at your site. Try adding this to your css. It roughly centres it. You may need to fiddle with padding a bit to get it perfectly centred.
margin: auto !important;
position: absolute !important;
left: auto !important;
Let me know how that goes.
Edit: Adjusted code based on discussions in the comment section below. Note, your max-width of the content and sidebar is 450px at the moment. You can adjust that max-width value to anything between your min-width and 450px (if you want to make it smaller than 450px wide). Just change the 275px value.