Just to clarify. Pro Tools 10 can playback your entire session from RAM. But as
Middleman correctly points out you do need ProTools HD and the Complete Production Tool Kit in order to do it.
In terms of RAM though I am sort of correct. The amount of available RAM for caching you have is approx 3G less than the total amount of RAM you have installed on your system. So if you have 16 Gig of RAM installed on your (Apple 64 Bit OS) then you do actually have 13 Gig available for this feature despite the fact it is only a 32 bit app.
You might be wondering how it's able to access memory beyond the usual 32 bit boundaries. The anwser is that it doesn't. By opening Activity Monitor on the MAC you can see that Pro Tools uses a second process called KMM_Server, which can run as a 64 bit process, communicating with Pro Tools to store and retrieve audio data for your session. But it still means you could playback an entire session from RAM even if the session needs well above 4 Gig to do it.
The next version (11) will more than likely be 64 bit anyway so its not a big thing. Playing back sessions from RAM is though, it is lightening fast and very responsive.