PSUs are confusing.
You need to look at the 12V power, which is what supplies power to the CPU and GPU.
A lot of cheap PSUs have rubbish power on the 12V rail, and loads of useless power on the 5V rail, and their power ratings are 'tweaked', aka not accurate or reliable by a long shot.
Unfortunately, there is no standard regarding the advertised power and the actual reliable delivery (kinda like laptop battery life, which are notoriously inflated). So you just need to look a bit more into it than any other components as a cheap PSU are a disaster waiting to happen.
In short, don't buy a rubbish PSU, even if advertised at 750W as they are most likely lying. Get a SLI certified PSU, with power to spare on the 12V.
options are OCZ StealthXStream II 600W, Be Quiet Pure L7 530W, XFX Core 550W, Corsair GS600, Corsair TX650. So around £60 mark.
I have reservations about the Elite Power 500W, can't find the Amp ratings on the 12V, not even on the manufacturer site, and it's efficiency is pretty low so not a good sign. CoolerMaster has a checkered history with power supplies. They top range are good, but their bottom range suck balls. However it should cope with a 5830. For anything higher, I would get a better PSU.