What components should I get?

[Your Computer]

What is a "bottleneck" exactly?

Pretend the liquid in a bottle is the data being communicated between components. There's a lot of liquid (data), but you can't bring it out of the bottle (component) all at once because of the bottleneck (ability to deal with or handle the data), and so it takes a while to fully empty the bottle. In other words, there is something inherent of the hardware or software slowing down communication. This is one of the reasons for overclocking, to help widen the bottleneck, however slight it may be.