I forgot to mention, you probably need to float the content div left also, that way both will be side by side.
Something you can check is the width of your container div, the one that holds your content and sidebar divs, and make sure that it's big enough to hold the width of the content div and the sidebar added together.
If it's not you can enlarge the container, or adjust the content and/or sidebar div/s to fit in the container.
Don't forget to include any left and right margin that is set on the content and sidebar, also check for padding on the container. Normally wouldn't expect that, but since the sidebar had 545 pixels of margin on it I'd be suspicious of the other divs.