I'm waiting for my own 5870 and I thought it be good to slow down memory to reduce heat production and therefore tune up the clock?!
The memory is not so important, right?
Exactly. Mining needs raw GPU computations and doesn't need to store a lot of data in memory. By downclocking the memory you reduce the heat, which in turn [may] allow you to increase the core clock a few more MHz and stay at the same heat levels. It's all a balancing act of gpu clock, memory clock, temperature, and Mhash.