http://www.scan.co.uk/products/500w-cooler-master-cm-rs500-psapj3-it-70-eff-uncertified-eps-12v-fan-atx-v23-psu
Is this a good PSU? I know you should be aiming for 80 PLUS but is 70% effiency ok?
A PSU is a pretty high point of failure. If it fails, it can fry all your components. It does pay to get something good. I see that you are using a UK website. You could get a better PSU from Ebuyer, or even Amazon. I'd recommend the XFX bronze rated 550W model. Widely available for around £50-55 delivered.
I remember reading something about Cooler Master PSU's sucking really bad or something.. I recently learnt that the PSU I got (Corsair CX750M) wasn't the best either so I went out and bought a HX850 which will go toward's my future CrossFireX'ing.
Go for 80% efficiency from brands like Corsair (Don't go for the CX models unless you don't plan on Overclocking etc) , XFX , Seasonic. If you're planning on using a single card set up , get a 550w or a 600w , you don't need to go over kill. If you plan on using CrossfireX or SLI later , get a PSU which is 750w or more and make sure that it is DEFINIATLEY 80% efficiency or something fry
:)