Recommended PSU for first time build.?

Hi all, I have recently decided to build my first rig, I think I've ironed out most of the details but one particular point I still don't fully grasp. Which PSU? The build I have in mind is.. i5 3570k sabretooth Z77 GTX 660 Ti (either Msi or Evga) memory undecided! Sea gate 1tb hdd ssd if I have the cash Cooling will probably be the Artic extreme + up to 6 case fans. I may add a second GPU and I will be looking into overclocking when my knowledge level improves so I'm ideally looking for a PSU giving me room to grow. Price ideally under £100, noise levels wont bother me as I have a Hpx headset sorted and my partner blasts music while I game anyway. Any constructive advise about the build welcome. Cheers Alex.