Hmm... Weird... I was reading a review online and they said they took it up to 612/800mhz and I just gave it a shot at 600/800 and did it with no problems and I'm running stability tests right now.
Hmm... For some reason 750mhz was the only one that gave me problems.
As is true in all OCs, YMMV. I used ATI tools to find a best OC for my card. It has saved me SO much time... I haven't heard of a "hole" in memory speeds but I'm sure you are familiar with FSB holes in OCing your CPU. I'm sure it's the same concept.