I've just got a Elsa Gladiac 920, it overclocks to 240/550 core/mem stable with stock cooling. At 245 core the card locks up on the second pass of 3DMark2001, i don't want to push the memory any higher because i've only had the card for a week and i don't want to wreck it!!
The Elsa comes with 3.8ns DDR-Ram, which is good for 530 non rated overclock, they have just conservatively clock the memory, or they could have gotten away with slower memory.