Someone Somewhere said:
I would guess that the 750W number is for CrossFire.
You'd be fine even on a good 500W PSU, and that is a very good PSU.
I suspected that it might be for CrossFire, it just didn't state that on the box which confused me a bit...
Thanks for your reply now, though. I have been sitting with it on my desk wanting to fit it all day not knowing whether I had to wait to buy a new PSU or not. Going to start fitting it straight away!