Sidebar Width/Centering

I’m new to wordpress(and web dev) and am attempting to edit a theme. My site is here: http://kevin.ultinets.net/dunn. I’d like my sidebars to be slightly wider, like here: finefinemusic.com, but when I increase the width, the sidebar moves to the bottom of the page. Also, i’d like the entire theme to be centered on the page instead of aligned left. I’ve tried everything i can think of, but don’t know what i’m doing wrong.

Any help would be greatly appreciated.