Here's how you test for a bottleneck:
1) Use FRAPS to monitor Frames Per Second in a couple different games with all the graphic settings maxed out.
2) Replay the same game(s) with the resolution and or graphic settings lowered. If you do not see an increase in FPS then your CPU (or other system component) is a bottleneck.
3) If you see a measurable increase in FPS, then you're CPU is fine.
Factors to consider:
1) The Game you play: Some games demand more or less from the CPU than others.
2) Clock Speed of your e8400: This CPU OverClocks very well. The faster the speed the less of a potential for "bottleneck."
3) Screen Resolution: The higher your resolution & graphic detail settings, the less noticeable a "bottleneck" will become, as your GPUs will be stressed more anyhow.