I notice you said that you would go SLI in the future, my only advice would be to do it within a year. Technology is advancing too quickly and just buying the new latest card at that time would be much better. Nobody looks at the 200 series and not many at the 400 series when going SLI, and that was just last year. I do realize that most people post that they will go SLI/Crossfire in the future, but i'm pretty sure that the majority don't.
Like you said you are already over, but from what you are getting and what you are listing for usage, I would recommend getting a 750W power supply.
It will then be enough to power the SLI of the 560 Ti and the overclocked 2500k.
Such as: Corsair TX750 V2 at £83.70
It's basically extra £33 now so that you don't have to spend £83 later to upgrade the psu when you SLI.
Hey, I am on another thread helping another user in a similar situation but I don't really want to start building for him because I am in Australia and he is in UK. The price of my build will be way off from what he get in UK. Can you guys help him out?