Here is Guru3D's power supply recommendation: GeForce GTX 980 Ti - On your average system the card requires you to have a 600 Watts power supply unit. GeForce GTX 980 Ti SLI - On your average system the cards require you to have a 900 Watts power supply unit as minimum.
http://www.tomshardware.co.uk/answers/id-3263837/xfx-psu-good.html
This thread says your psu although meeting the wattage requirement for the gtx 980ti, is not very good. The way to find out is to find a quality 750w psu for some additional margin, is the way I would go, but it probably ain't cheap.
http://www.tomshardware.co.uk/answers/id-2809876/whats-power-supply-980.html
These guys are recommending an evga supernova 650 current price has gone up since their convo it's now closer to £90.
Best advice is don't try and overclock it especially since you seem unsure of what you're doing and also have not listed your system specs
http://www.tomshardware.co.uk/answers/id-3886538/overclocking-make-pci-slot-fail.html
Any time you overclock you are risking your system hardware. Someone else with a 250w tdp gpu lost a motherboard t'other day. I hope you leave here persuaded not to try and overclock it, until you have developed some understanding of the pc.