do I need a 750 watt psu for my pc build

Yes, that mobo will do well.

Be advised that the build I put together is for gaming and not for some other use like video editing. It makes a difference.

PCPartPicker part list: https://pcpartpicker.com/list/LLMzjc
Price breakdown by merchant: https://pcpartpicker.com/list/LLMzjc/by_merchant/

CPU: Intel Core i5-7600K 3.8GHz Quad-Core Processor ($233.79 @ SuperBiiz)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($29.99 @ Newegg)
Motherboard: Gigabyte GA-Z270X-UD3 ATX LGA1151 Motherboard ($149.79 @ OutletPC)
Memory: Corsair Vengeance LPX 16GB (2 x 8GB) DDR4-3000 Memory ($109.99 @ Newegg)
Storage: ADATA Premier SP550 240GB M.2-2280 Solid State Drive ($69.99 @ Newegg)
Storage: Hitachi Ultrastar 7K3000 2TB 3.5" 7200RPM Internal Hard Drive ($61.39 @ Amazon)
Video Card: Sapphire Radeon RX 480 8GB NITRO+ Video Card ($234.98 @ Newegg)
Case: Cooler Master HAF XB EVO ATX Desktop Case ($94.04 @ Jet)
Power Supply: SeaSonic 650W 80+ Gold Certified Semi-Modular ATX Power Supply ($77.99 @ SuperBiiz)
Total: $1061.95
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-03-22 08:56 EDT-0400

Just thought I would provide a list with everything coming from Amazon. Had to change the GPU and PSU.


PCPartPicker part list: https://pcpartpicker.com/list/CdKzjc
Price breakdown by merchant: https://pcpartpicker.com/list/CdKzjc/by_merchant/

CPU: Intel Core i5-7600K 3.8GHz Quad-Core Processor ($248.10 @ Amazon)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($39.30 @ Amazon)
Motherboard: Gigabyte GA-Z270X-UD3 ATX LGA1151 Motherboard ($154.86 @ Amazon)
Memory: Corsair Vengeance LPX 16GB (2 x 8GB) DDR4-3000 Memory ($118.10 @ Amazon)
Storage: ADATA Premier SP550 240GB M.2-2280 Solid State Drive ($83.67 @ Amazon)
Storage: Hitachi Ultrastar 7K3000 2TB 3.5" 7200RPM Internal Hard Drive ($61.39 @ Amazon)
Video Card: Gigabyte Radeon RX 480 8GB G1 Gaming Video Card ($239.48 @ Amazon)
Case: Cooler Master HAF XB EVO ATX Desktop Case ($113.70 @ Amazon)
Power Supply: SeaSonic S12II 620W 80+ Bronze Certified ATX Power Supply ($61.61 @ Amazon)
Total: $1120.21
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-03-22 09:02 EDT-0400

Ask questions, better to answer them now than later when you are regretting a bad build.

No, actually you don't really need an 8GB card, not much performance difference at this point. It was more of a future proof as developers take more and more advantage of VRAM.

No, perfectly acceptable to just run them at stock speeds. But like a car, you change your build based on whether it will be used on the street or the racetrack.

I am scared to answer this question

There are several different methods to do this and far more opinions on whether they are accurate. I am in the camp that power requirements can be modeled and predicted, after all, in engineering terms they are just power systems operating on known physical principles. We design and build far more complex systems than a home computer. It is just that there a number of variables for each system that makes it difficult to determine a global estimate for that configuration.

It should be noted that you never want your system to be operating anywhere near the max power delivery capability of your PSU. Power demands wobble up and down from the average from one millisecond to the next, so you need a good buffer zone. And actually some power system components operate best in the 50-75% zone of rated load capacity.

Easiest estimate is to just go with the GPU min power requirement listed by a reputable source, like RealHardTechX. They list min requirement as 500W. This would be considered a safe, global overestimate for all RX 480 installations.
http://www.realhardtechx.com/index_archivos/Page362.htm

Next you might have a measurement of system requirement and subsequent recommendation like Guru3D did (450W)
http://www.guru3d.com/articles-pages/msi-radeon-rx-480-gaming-x-review,5.html

For this estimate since the CPU and GPU consume most of the system's power you can take the power requirement of your GPU plus your CPU and multiply it by 1.5 to account for other components and needed buffer zone. Here we have

RX 480 = 150W TDP (Thermal Design Point)
i5-6600K = 65W TDP
Wattage = (150+65)*1.5 = 323W

Which is a lot less than the min from RealHardTechX.

Next, you can use a power supply calculator, such as PCPartPicker or Outervision

PCPartPicker Suggested PSU Min = 365W
Outervision Suggested PSU Min = 336W

So you can see that standard recommendations tend to be much higher than calculated values. This is because it is much better to be slightly over-powered than slightly under-powered and they want to be conservative. But I do not believe that it is worth it to try to determine a close limit for wattage, like I said, power pulls wobble all over the place. Even the process of power measurement has its issues and errors.

Last comment is that all of this is at stock, overclock and all this goes out the window.