Just going to tack on a little easier, more applicable way to detect "bottlenecks."
Run FRAPS or some other FPS counting software.
Run each of your games. Set the resolution to native and the settings to highest. All the way to 11.
See what FPS you get as you play for a little while. Once you have a good idea of your average FPS, drop the resolution to lowest. If your framerate did not increase, you have a CPU bottleneck and overclocking would help the game perform better.
Alternately, if you see a boost in FPS, then you have a GPU bottleneck. Alleviating this bottleneck is as easy as turning down the eye-candy. Overclocking your CPU will not help if your fps is determined by your GPUs.
Keep in mind, if you're getting 60+ FPS with a CPU bottleneck, chances are overclocking won't help you. (As you would have most likely reached your monitor's refresh rate.)
This kind of testing is the only way to
know if you have a bottleneck or not.
Geofelt's suggestion to mess around with # of enabled cores would also be helpful if you were trying to gauge how beneficial a CPU upgrade would be.
Final note, to ensure a reliable "testing environment," make sure your computer is free of malware and any unnecessary programs that may be wasting CPU cycles.