I'm using a theme based on 1140px grid and I'd like to make the content area narrower while making the sidebar wider (currently it's 250px). How do I ensure everything stays proportional? I'm looking at something in the region of 300px for the sidebar.
Sounds to me like you aren't happy with the theme and you need something new.
Gridpak is an excellent responsive grid generator that may save you a lot of pain and suffering. Enter your column count, padding, gutters, and simply drag a pixel-snapped ruler to set the total size then download your files.
Gridset is a more in-depth, professional grade responsive grid generator. If you're up for that, this one will also cost you a little (very little). I haven't used it since private beta but it produced beautiful responsive code then.