Example on price difference between a 500w Corsair CMPSU-500CXV2UK Builder Series CX500V2 - 80 Plus Certified Power Supply: Amazon.co.uk: Computers & Accessories
and 750w Corsair CX750 Builder Series CX 750W ATX/EPS 80 PLUS Bronze PSU: Amazon.co.uk: Computers & Accessories
For the sake of £20 ish the 750w is the better option as it will cover your needs now aswell as some future upgrades. So you won't need to buy another next year. The Psu is the most important component and should never be skripmed on. Always buy a well known and reliable brand..