A bottleneck is when your performance is limited specifically by one part. For example, if your CPU was being pushed at 100% but your GPU was only at 50% of it's capacity, your performance is limited by the CPU and you have a CPU bottleneck. I can't really point out where the bottleneck in your computer is, but it's usually the GPU in gaming.
As for the 2x4GB vs 1x8GB debate, a 2x4GB array will be faster because the computer can utilize 2 channels rather than only 1 to move data in and out of the RAM. Additionally, you are unlikely to need more than 16GB memory within the next 5 years (the longest I personally would go without upgrading) and would need to get a new operating system (Windows 7/8 Home Premium is artificially limited to 16GB; You would need Win7/8 Pro).