Jez,
not that I'm not appreciating your answer, but "I believe" or "I think" is of no use for somebody who wants to build a new board.
This is the reason why I called the support poor, because I didn't get there a precise answer, too. No application note says explicitely/exactly which RAM-speed is needed for 75 MHZ operation.
One the other hand: no body else seems to know it, because of no answers.
If anybody in the world knows it exactly please tell me, thank you!