The half banner could be a problem depending on the heading you are using above it, AdSense explains here why. You could consider making a wider sidebar to include a 300x250 instead of the 200x200 you are currently using. It performs better. Use image and text ads for all your units, like Mightee said.
I have performed different google adsense placement experiments, Please see following link, It might help: http://techbrij.com/576/google-adsense-tips-placement-experiments