Glossary:Bottleneck
A bottleneck is a situation that arises when an underpowered component limits the performance of another component. For example, running Battlefield 3 on a single core CPU will severely impact the frame rate. Another example is not having enough RAM to run a game and the operating system, resulting in slowdowns.
Unfortunately, there is no surefire way to gauge if you have a bottleneck, nor what is potentially causing it. Your best bet is to find a pc gaming forum and ask around if you think you may be experiencing a bottleneck. A good rule of thumb is that if your CPU/RAM/GPU is 2 years or more older than the other components in your computer, then it may be causing a bottleneck. Use common sense though, if you put a new $400 graphics card in a 5 year old computer, chances are you will experience one or more bottlenecks.