"Bottleneck" simply refers to one component having to wait on another component to complete it's task before passing on the next one. Think of it like people waiting in line at the bank. There are four tellers and ten people in line. In this example, the tellers are the bottleneck as the people in line have to wait until the next teller is available.
All systems bottleneck at some point; be it the processor, graphics card, RAM, or hard drive. There is no getting around it, but fortunately, unless it's a serious bottleneck, we don't notice it.
As far as your rig is concerned, you could probably put any single (and probably most dual) graphic card solution in there and not worry about it.