I have a choice between...
an XFX core pro PSU 550W with 53A on the 12V rail 5 year warrenty
or an OCZ ZS series 650W with 46A on the 12V rail 3 year warrenty
the OCZ is £2 more so basically the same price
or would a coolermaster GX 650W with 52A on the 12V rail 5 year warrenty be the best option? (it's £10 more than the others) I'm not sure how reliable and quiet CM's PSUs are though.
Robi: Beyond buying a good quality PSU, the most important factor in selecting a PSU is the combined amperage on the 12V rails...not the wattage. I would go for the XFX (Seasonic is the OEM) over either of the OCZ PSUs.
I ran some estimations of your power consumption, and your max power draw would be about 386watts, note that that is just an estimate, and the recommended psu for your setup is about 430-450watts, so you are well within the limits. So you should be good to go with the XFX psu, unless you want something else. Guru3d recommends a 550watt psu for two of those cards, and but that is with overhead, people are running these cards on 450 watt or less psu's without problems, so you should be fine with 550watts. Both cards use only 159watts on full load, thats insanely little.