My build is currently http://pcpartpicker.com/p/cZY3cf but I do not know what a good power supply is. I will be ordering the 1080 GTX when it comes out and don't know what is good for that sort of build.
A 500-550 watts would be fine for one GTX 1080, and with a 750 watts yoou can SLI 2 GTX 1080s
a evga p2 650w would be fine for that build
Any Antec, Seasonic, or XFX unit would be good.
EVGA's G2/B2/P2/GQ/GS series are great, and Corsair RMx, RMi, AX, and HX are solid.
As for wattage, go with what Pegotico said.
Sadly, not all XFX units are good.
I would recommend watching reviews.
I must say in the UK the XFX beats the other brands in price so its a good thing to learn about.
XFX stuff is made by SeaSonic so it's prett much good all around
The XFX XT is not one of them sadly.
They offer a 2 year warranty and the information about them on the website, the product's box and the PSU itself are all different and some of it makes no sense.
Aside from that, the rest should be fine.
I guess the XFX XT is the quivalent EVGA's NEX.
The XT is now confirmed by XFX to have a 3 year warranty.
It's best likened to EVGA's 400W - The lowest end from both, not having complete input filters, stuff like that.
1 GeForce GTX 1080 = 180 watts
1 GTX 1080 : http://pcpartpicker.com/part/evga-power-supply-220p20650x1
2 GTX 1080s : http://pcpartpicker.com/part/evga-power-supply-220p20750x1