If the power supply is acting strangely, for example with strange sounds, I would suspect you are stressing it too much... but a single 5870 could easily run with a 600W power supply.
It's probably the quality itself of the power supply that is at stake here. I would probably not take a chance and change it for something else.
You can get a good OCZ power supply for 50$. If you are thinking about going Crossfire later, you can get a Corsair for 100$.