[Build Help] Considering Upgrading but I am not sure whats holding the PC back. If GPU, would the new one be worth to buy?

Hi Folks,

i am considering upgrading my machine, since i am not satisfied with the current amount of FPS in Games.

Currently this is my set-up:

CPU HexaCore AMD FX-6100, 3600 MHz
Motherboard ASRock 970 Extreme3
RAM 8171 MB (DDR3-1333 DDR3 SDRAM) 2x4GB
Graphics Card AMD Radeon HD 6700 Series (1024 MB)

I am getting like 30FPS in Borderlands 2 (higher details, 1920x1686) and about 25FPS in FarCry 3 (medium settings, depends in which in-game direction i look :D). Guild Wars 2 has around 25-40FPS.

I've read that AMD FX-CPUs aren't the best for gaming, but i guess the Graphics Card is the weakest link. So I was looking to buy a GeForce GTX 660 Ti card.

So my Question is: Is the CPU or the GPU holding back my machine. Would the Upgrade result in a much better performance?

Best Regards