I was wondering the same thing myself....I have some Micron D9GMH chips in my Ballistix DDR2-800s, and I run 2.25V 24/7 @ DDR2-833, but this isn't my max.
My modules are pretty much room temp to the touch when idling, and I have a hard time feeling any heat when running Orthos blended or HCI Memtest....I have active cooling and excellent case cooling....