Could simply be the PSU not being able to power the GPU properly, causing the corruption you see as blue lines.
OCUK will test the GPU on a quality PSU. I believe they'll return it to you if it works perfectly for them.
If you ever want to play future games on your PC, you'll inevitably have to upgrade many of your components, one of which will be your PSU.
It could be worth going ahead with the 500w Superflower PSU - it might fix your issue. If it doesn't, then RMA it and you'll get a replacement/refund which you know will work perfectly with your new PSU.
Up to you buddy, goodluck with whatever you decide
P.S. Alien Isolation is a fantastic game, if your an Alien fan I'm sure you'll enjoy it, hope you get to play it soon.