The 5850 specs are:
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)
For twin 5850s I would recommend a 650W PSU with four PCIE 6pin connectors. Reliable power makes or breaks the system - if you get a crappy power supply you will at best have flakey lockups/performance problems and could possibly damage other components.
For RAM, DDR3-1333 Cas7 should be fine. Unless you have a specific reason for 8GB RAM, 4GB would probably be plenty. 8 is slower and uses more power if you never use 4 anyway.