"Our tests demonstrate fairly little difference between a $225 LGA 1155 Core i5-2500K and a $1000 LGA 2011 Core i7-3960X, even when three-way graphics card configurations are involved. It turns out that memory bandwidth and PCIe throughput don't hold back the performance of existing Sandy Bridge-based machines. "
http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-4.html
This means that your CPU is not the bottleneck here. Let's compare your 980 to a 1080.
http://gpu.userbenchmark.com/Compare/Nvidia-GTX-980-Ti-vs-Nvidia-GTX-1080/3439vs3603 tells me you may not get a big bump in frames but more details. If you are trying to go 4K gaming, this isn't enough yet. Stick to 1080p gaming for now.
I just saved enough money to buy an Asus GeForce GTX 1080 Founders Edition (8GB) (I currently have the GTX 980 4GB) but I'm also thinking of getting a new CPU since mine is a bit old (i7-3770) and replace it with an i7-6700K 4.0 GHz but I can't get both at the same time. Should I upgrade my GPU or CPU?

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