It should perform like a 4850. The 4850 and the 4870 share most of the same everything, but the 4870 has faster clocks and DDR5. Seeing as they changed the memory, make sure they didn't change the clocks as well.
Well, any 500W PSU from trusted brand like Corsair, Antec, OCZ, Seasonic, PCP&C, SilverStone are more than enough to run single HD4870.
I know they're expensive, but you will not regret it later, those are high quality PSU.