well, 500watts is plenty for your system, but it would seem that you have a low-quality power supply, unable to supply the wattage it's rated for. You can read a review of a Thermaltake PurePower at HardwareSecrets.com. You'll see that the power supply was rated for a much higher wattage than it could actually deliver, and had terrible ripple suppression even at loads it was able to run.
As for a new affordable PSU, the OCZ StealthXStream 600watt power supply is currently on sale for $50 at newegg.com, with free shipping. That's a high quality power supply that has more than enough power to run your computer, even if you were to overclock. That power supply was also reviewed at HardwareSecrets, with much more positive results. In fact, they found that the OCZ StealthXStream 600watt power supply was actually capable of putting out a full 728watts, amazingly enough. I'd recommend getting one of those soon, before the sale expires or the price goes up.