Performance Question: Where is the Bottleneck?

My frame rate begins declining the more monsters I see on screen, sometimes down into the 20s or teens. Does this mean that it is a CPU issue, rather than a GPU issue?



Just wondering if I upgraded my graphics card, would I get higher frames per second. Or do I just need a better processor.



(Also trying to plan for Killing Floor 2.)



Thank you.



System specifications:

Ubuntu 12.04.4, 64-bit

Unity Desktop Environment (vanilla)

Asus M4A78LT-M LE Motherboard

AMD Athlon II X4 645, 3.10 GHz

8GB RAM

Nvidia GeForce GT 240

Nvidia Proprietary Driver (Version 331.38)