Would 1024MB modules be 3.9us? Everest is reporting them as 7.8us but that doesn't make much sense if 256MB are 15.6us and 512MB are 7.8us.

And have we concluded an effective way of calculating a CORRECT tREF value yet? Is it MEMCLCK ratio x refresh rate (200Mhz 1:1 x 3.9us)?