I run my Sapphire 5830 core at 966 MHz and the Memory art 325 MHz.   I actually get faster hashing speed when the memory runs between 300 and 350 MHz than the default but it crashes if I launch a game.   I think that the sweet spot is tightly related to your core speed.  Some people are obtaining better performance at other mem speed but 325 works best for me.

I haven't changed the voltage, MSI afterburner won't let me do it on this card.