At least 500W, but I would go higher.
I'm currently still finalizing my power supply & i have no idea which power supply should i get.
Motherboard: Asrock h170m pro 4
CPU: i5-6600
GPU: Asus strix gtx 960 4GB
RAM: kingston fury 2600mhz 8gb
SSD: samsung 850 evo 250gb
HDD: wd 1tb blue
my expectation for power supply is energy saving & effective for gaming performance with cost saving. my budget for the power supply would be $110
At least 500W, but I would go higher.
a 500W power supply is enough, but if you are going to be upgrading the pc then get a 750W
650W to be on the safe side.
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650W PSU just to be safe(you would be able to overclock your card if you're planning to). I've got the EVGA NEX650G, it's pretty decent, cerified as gold so you should be fine with the setup you're having. I'm running i7-4790k and GTX 970 off of it. Good luck!
I would go for a 650W as i've had a build that had an issue with the psu being too low and it caused me a load of issues for a while until i figured out that my GPU wasn't getting enough juice lol