Not to rain on anyone's parade but upgrading your hardware or overclocking isn't going to drop your CPU usage unless you are hitting an FPS limit (Edit for completeness: or adding more CPU cores). All you're going to manage to do is raise your FPS, making your PC work harder, and what you're asking how to do is free up CPU time... In order to free up some CPU time, it sounds like all you want is to drop your FPS limit by some degree. The FIRST question I would ask is what do you have your FPS limits set at (assuming you're using a solution that is capable of doing this), and what FPS are you getting? Then it's a no-brainer as far as adjusting the limits. Your foreground framerate really only needs to be 30+, and background can probably be as low as 20 -- it's also based on personal preference so I also expect someone to reply and say they wont play without 60 in the foreground, but note that DVD playback is about 30 fps. Higher framerates are more useful in other types of games (first-person shooters and such) where precision in turning and movement is paramount.

Hope this helps.