I would recommend at least 850 Gold or 1KW if you want to OC.
She use 25-50W more than normal GTX780Ti because OC and if you want to OC more with Voltage Tool can use even 150W more than normal GTX780Ti on air or water-cooling. On LN2 who knows how many power need.
GTX780Ti Classified 8+8 pin
GTX780Ti K|NGP|N Edition Classified 8+8+6 pin
 
I don't say 850-1000W only because graphic card, some space should be at least 100W and space for another hardware.
Imagine if you change for 2-3 years CPU and next need 200W, 3-4-5 HDDs collect during years and that's it...
 
Example if we talk about excellent 750W Gold PSU on 700-750W 12V rail will not be any more as on 500-600W.
If we talk about some II or III class PSU that will happen on 600-650W and could cause instability.
 

pierrelanson
Oh crap sorry i meant the K|NGP|N edition but i was not thinking of overclocking i'm looking into this card as i found it for only £300 and wondered if my pc could run it. If you need all my specs they are here;
CPU - i7 2700k @ Stock clocks
CPU Cooler - Xigmatek prime sd1484
Motherboard - Asus P8Z77-V LX
Ram - Corsair Vengeance 8gb
Case - Corsair 400r
PSU - Xigmatek 750w 80+ Bronze

750 watts is plenty to run a single K|NGP|N GPU. However, the exact part number of your PSU would be helpful.