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Love these threads .
What CPU do you have now op? You could be just wasting money if you have a decent CPU already
A GPU upgrade will benefit you more.
If you feel the need to upgrade something then aye, GPU. With freesync I'd look for offers on a Vega 56, not checked out their prices in a while but I hear they're coming down again after the worst heights of the mining boom.
Still quite expensive to improve GPU from where you are unfortunately
Personally I'd stick with an AMD card for the sake of freesync rather than any of Nvidia offerings but if you were tempted that way you'd be looking at a 1080 roughly.
Not sure if you have mentioned it but what refresh rate are you running on the monitor @ 1080p
What RAM and motherboard dod you have OP?? If the RAM is fast enough,and the motherboard can handle it a Core i7 7700 or Core i7 7700K would be a much easier upgrade to do.