David, I just want to make sure we're thinking about the same thing here. You really know your stuff when it comes to digital music so maybe you're talking about something other than what I'm thinking here: I use Pro Tools 10 and you have the option to load the entire session you're working on into RAM, including audio tracks. I do it all the time and with 16GB of RAM (you can allocate up to 2GB less to Pro Tools than your system has). It puts every audio file that's in the session's audio folder into RAM, even if the timeline doesn't reference that audio. If you don't have enough RAM to cover the needed size of the session, it will load as much as it can into RAM based on the amount you've allocated, and you'll still see significant performance increase. But I've never come close to maxing out... I typically take up half of my RAM with my sessions.
Is this what you're thinking, or are and the OP referring to something different than my understanding?
PS - granted, most of my sessions are 4 to 5 minute songs. When you start talking about movie scores and talking sessions, you'd certainly run out of RAM quickly!
Yes, that's what I'm talking about. It's a lot more snappier.