With
two 980 Ti cards, 750W is cutting it a bit thin!
You didn't tell us everything the PSU needs to support. For example, what type and how many case fans? What type of CPU cooling? Will be there be any connected USB devices (besides keyboard and mouse). Any extra drives? What about any planned (or unplanned) upgrades - like two more sticks of RAM? A DVD burner or a hard disk drive for storage?
While it is very rare for the graphics solution, the CPU, the RAM, the motherboard and the drives to demand maximum power at the exact same point in time,
you MUST assume it WILL happen when sizing up your power supply requirements!
I definitely am
NOT a fan of "overkill" when selecting a PSU. That's just a waste of money, at best. But it is a HUGE mistake to buy an underpowered PSU too. An underpowered supply will result in unstable operation, crashes, excess heat in the supply and perhaps a PSU fan that must run at full speed (and loudness - I
really hate fan noise!) much of the time - and that's with a quality supply. A no-name generic budget supply may overheat resulting in much worse things happening than just crashing the computer!
DO NOT JUST GUESS! You need to do your homework and research the maximum demands for EVERY component your PSU will support. Then total them up, add a little headroom for good measure, then select a quality supply that meets those potential needs.
Or, use a good PSU calculator and there is only one, IMO, and that is the eXtreme PSU Calculator. As I said above, the right way to determine the proper size needed is to research all the components, determine their
maximum power demand, add them all up, and that is the
minimum supply you should get. The
eXtreme OuterVision PSU Calculator does this for you. It will calculate your minimum needs and recommend a suitable size for those needs.
Plug in all your current components. Be sure to plan ahead and include all the hardware you think you might add in 2 or 3 years (extra drives, hungrier CPU, more RAM, etc.).
I recommend setting CPU utilization to 100% and Computer Utilization Time to 16 hours per day. These settings will help compensate for component aging, and add a little extra padding to the results. This will also result in a little cooer and quieter operation. Did I mention I hate fan noise?
As seen
here, when I plug in the components you list above, then add a DVD drive, 3 x 140mm case fans, and add 1 x 7200RPM SATA drive, the maximum potential load those components could put on your supply is 800W. So the calculator recommends an 850W supply. And I agree with that. But note I did
not factor in any overclocking.
Note that no calculator wants to recommend a PSU that is underpowered so they all pad the results, some more than others. The eXtreme OuterVision calculator is and can be the most conservative for 2 main reasons. (1) They have a team of researchers on staff constantly researching components for us to keep their extensive databases accurate and current. And (2), it is the most flexible and has the most extensive databases of available options we can enter. This allows it to factor in all possible components to accurately calculate our needs rather than guess.
Want more proof 750W is not enough? Okay. See
this Guru3D review and note where it says the card can demand a maximum of 250W and with an "average" system and
just one 980 Ti, they recommend 600W minimum. You are using two cards.
Now doubt 850W is big. But it is not "overkill". But more importantly, it is not underpowered either.