I'll probably get shouted at for this, but assuming your PSU is delivering the correct amount of power, a new one, no matter how expensive is unlikely to gain you FPS/better scores.
What you MAY notice is fewer crashes, lower temperatures and generally SMOOTHER performance.
Most people just buy a decent PSU because it's silly not to, when you consider what you spend on other components and because of the damage that one could potentially cause when blowing up!
I've just got an 'EzCool' - whilst it's not the most amazing ever, it does have a 12cm fan, and 30amps on the 12v - being as my computer is not massively 'uber', that'll do quite nicely - it's quiet and it works...for now!
Cheers,
James