Currently RAM speed is still designated by the Motherboard. Also, At the Moment the DDR400 (PC3200) is the highest speed adhering to the JEDEC standards. Anything higher is not officially supported by anyone (I don't think) but will work, and is useful for overclockers. Anyone not overclocking will get no added benefit of say PC4000 RAM, because the PC4000 DDR533 RAM will just run at 400mhz (DDR400 PC3200)
This is not to say that in the future S939 motherboards won't support higher, but at the moment they do not.
Please correct me if any of my information is incorrect.
Why you should care about the PATRIOT Act.