I suspect that there is something very screwed up with your installation. Your GPU has not been supported by fglrx in over five years. The fact that Ubuntu insists on installing fglrx is a pretty big indicator that something is not right.
I could be wrong and the the high CPU usage could be related to the fact that you don't have direct rendering working with the radeon driver. What's the output of 'sudo modprobe radeon' ?
Adam
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