No a 500W PSU will not be enough for two 5850s. Do you have a system budget and a PSU budget? You should try and get the best power supply you can, as a bad one can ruin other components, meaning that you will spend more in the long run. A 650W PSU is powerful enough for two 5850s, altho they don't usually have enough connectors for more than one (a 5850 will need two 6pin PCI-E connectors). It is usually 750W PSUs that do, however you can use adapters with the 650Ws.