It's not just wattage that is important, but also amperage and quality of a PSU's components.
The quality PSU's on the market today are made or sold by Corsair, Seasonic, PC Power&Cooling, Enermax, XFX.
The two most power hungry components of a build will be the graphics card and CPU.
Nvidia recommends a PSU with a minimum of
500 Watts total power and a minimum of
30 amps on the +12V Rail for a
system with a Single GTX 560.
Here are a couple PSUs that should work just fine:
Corsair CX500M, made by Channel Well Tech, modular, 38amps +12v
http://www.amazon.co.uk/Corsair-Builder-Series-Modular-Certified/dp/B00ALK1GFC/ref=sr_1_4?s=computers&ie=UTF8&qid=1374446264&sr=1-4
Seasonic (arguably the best PSU manufacture), Made by Seasonic, 550W, 45amps +12
http://www.amazon.co.uk/Seasonic-SS-550RM-550W-Power-Supply/dp/B00918MEZG/ref=sr_1_10?s=computers&ie=UTF8&qid=1374446321&sr=1-10
Sorry, couldn't find a distributor in Europe other than Amazon UK.