Someone already made a chart of that, but Ill damned if I can find it. IIRC the bottom line was that ~200-400 MHz was the sweetspot, performance tapering off slightly in either direction
i found a chart but it only showed that 300mhz was the spot & only for one card.
i was thinking a table with many cards & values..
http://smpake.com/data/img/5770mem.png