Can't decide if I need to upgrade my PC or not?

I made a mistake a while back and bought a pre built computer. Here's a link to it on amazon. "http://www.amazon.com/Lenovo-X315-Gamin… Make sure you look at the a8 version. I was new to PC gaming and thought it was a good deal, but I ended up not being able to play most the games I had bought so I replaced the graphics card with a 2gb gtx 960 and the power supply with a 600w. And I'm still not getting the type of performance I want to be. I feel like it should be better. I see my fps drop down to 20 in games like fallout 4 and just cause 3. So I thought huh maybe my cpu is the problem so I looked at the i5 4690k and the i7 4790k and I didn't see a huge difference in performance I mean not $100 worth at least. And I couldn't find a video comparing my cpu the a8 7600 and the i5 4690k so I looked it up on cpu boss and my cpu I have now had a higher score. So my question is what do I need to upgrade. As far as I can tell I would think I should be getting around 40fps on medium setting in most games at least. I think I could be happy with 40fps for now. Any ideas would help.

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