How did you go about working out it was your PSU?
Process of elimination really. Swapped the gfx card for an old one, same prob, ran with one mem stick then the other, same problem, removed sound card, same problem and so on. Eventually I dug out a ropey old emergency PSU I had tucked away in a cupboard and tried that, and despite the PSU being horribly under-rated for my system it ran 100% stable so it pointed to the PSU being the problem. Got that replaced and everything back to normal.
As for your fans, I'm sure they're going to be blowing the right way but worth checking of course. The side fan could legitimately run either way, either as a blowhole to extract hot air, or as a method for blowing extra cool air onto the GPU. Which is best depends on the airflow inside your case (I'd probably leave it the way it's already mounted).
It's unlikely to be your CPU overheating if you have an aftermarket cooler and it has been fine up to now (assuming it is still properly fastened down).