What graphics card are you planning to get? Also where are you getting the 500 watts from?
Most likely the 500 watts is for the complete system
not just the graphics card.
What is more important than the wattage is the Amps on the 12v rail. You will need a good quality power supply as there are power supplies that claim they are 750w + but actually half of that.
You can take a look here
for SLI GTX 580 power consumption under load.
Here is a power supply calculator
to get a idea of how much power you will need. I estimate a good 850w power supply will be enough for two cards.