500w is ample power.
Thing is don't just buy a cheapo nasty power supply just because of the hefty wattage, Make sure it's good quality or you're wasting you're time.
Hope this helps a little
My Setup -
AMD 64 3200 +, 256 mb Geforce 6600, 1GB PC3200 DDR400 RAM, 160GB SATA 7200RPM HDD