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FrostinFire I'm reasonable sure that an EVGA P2 650/750/850 will fit in your case based on the video I hyperlinked earlier. The guy successfully installed the Zotac GTX 970 along with a ThermalTake Smart M850W PSU, which has dimensions of 3.34"(H) x 6.30"(L) x 5.91" (W). The EVGA P2 650 has two of the same dimensions, with only the length being 0.20" longer for a total of 6.50".
Thoughts: The EVGA P2 line is one of my favorites and I personally own several of the EVGA P2 850 PSUs. They are fantastic: EVGA customer support, manufactured by SuperFlower, tier 1 on Tom's Hardware PSU tier list, 10 year warranty, fully modular and platinum efficiency. With that said, it's expensive
IF you only plan to upgrade this particular computer and never see yourself upgrading beyond a GTX 1080. If that fits your scenario then a SeaSonic S12II 520 will handles your needs (
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151094&cm_re=SeaSonic_S12II_520-_-17-151-094-_-Product ).
However, if you plan on keeping your new power supply beyond the life of this computer, and can see yourself one day upgrading to a GTX 1080 Ti (titanium level successor to the 980 Ti that hasn't been officially announced yet) then the EVGA P2 650 would be a great way to go. I'd even go as far to say that you might consider the P2 850, as it's currently only $10 more on NewEgg, and would give you the future upgrade option of two GTX 1080 Ti's in SLI, if you ever built a new machine; your current one only has a single PCIe x16 slot. For only $10 more, "in for a penny, in for a pound" comes to mind.
EVGA P2 850
$109.99 after $20.00 rebate
http://www.newegg.com/Product/Product.aspx?item=N82E16817438056
SeaSonic S12II 520
$64.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151094&cm_re=SeaSonic_S12II_520-_-17-151-094-_-Product
Btw, I retract my earlier suggestion that you toss the OEM 300 watt power supply after you swap it out. You might want to hold onto it for the future when you're ready to step-up to a new computer. You'd have the option of transferring the EVGA (or whichever) PSU to a new build and then you could re-install the OEM 300 watt unit and sell the machine to someone on Craigslist or Ebay.
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power requirements for graphics cards ***
http://www.realhardtechx.com/index_archivos/Page362.htm