Will the GTX 960 work with this prebuilt pc?

Solution
A 750ti will be pushing your luck. A gtx960 will not work, period. That unit is supposed to be 600w yet only has 20A on the 12v rail? A decent 600w unit will not only have upwards of 45A on the 12v rail, but at least 2x 6+2pin pcie connectors. How Orion actually came up to a 600w total is beyond me. Given the specs, it's more likely a 300-325w unit and given the nature of those Orion units design and quality, it's more likely a good candidate for early death at any load past @50%.

https://youtu.be/f6snWfd1v7M

Sorry, but in order to make a profit on a cheap case with included psu, some serious amount of corners must be cut, at best that psu alone is worth @$5. A decent psu will cost about 2x what you paid for the case + psu.
A 750ti will be pushing your luck. A gtx960 will not work, period. That unit is supposed to be 600w yet only has 20A on the 12v rail? A decent 600w unit will not only have upwards of 45A on the 12v rail, but at least 2x 6+2pin pcie connectors. How Orion actually came up to a 600w total is beyond me. Given the specs, it's more likely a 300-325w unit and given the nature of those Orion units design and quality, it's more likely a good candidate for early death at any load past @50%.

https://youtu.be/f6snWfd1v7M

Sorry, but in order to make a profit on a cheap case with included psu, some serious amount of corners must be cut, at best that psu alone is worth @$5. A decent psu will cost about 2x what you paid for the case + psu.
 
Solution
Well the exact needed kinda depends on the gpu, and that depends on your needs and budget. For the price of a 960 you could get a gtx 1050ti and be quite comfortable with a good 450w unit, whereas for a little more, the Rx470 would also be a good option at 450w. If going for slightly older cards, say from ebay, you'll need to start thinking about a good 550w. Most of these are subjective, recently there's been many 650w models actually cheaper than the exact same model in 550w.

Seasonic S12-II or M12-II 520w I, Antec HCG-M 520w, Evga GS or G2 550w, XFX TS/Pro 550w (not XT) Corsair CXM 450W/550W are all very good, which ever is cheapest on sale would be best value.