I just tried using a ram disk with VDMX (from the donelleschi site recommended by another user on this forum - this just provides an interface for Apples native ram disk funcionality I think) and can't say I've noticed any difference at all?!
I usually play my movies from an external firewire800 drive and load them into the aux tabs for ease of use during performances (when the interface response rate drops you need to be able to see your clips without using a drop-down menu).
When using the ram disk I still found there was a slight delay on changing clips. 'Slight delay' being way too long of course!
Using the pre-roll function in the movie player is really good for changing clips smoothly and I expected that having the entire clip 'pre-rolled' in a ram disk would be even better - or at least as effective.
I can honestly say I found no difference whatsoever. Strange.