Need info on RAM needed for REAPER

Does REAPER only allow us to use a maximum of 1.5GB of RAM? An error message from Kontakt keep on saying that it could not allocate enough memory and the application is likely to become unstable.
It happened when my RAM usage exceeded 1.5GB. Can someone give me complete details about this problem? It is confusing me because I still have enough memory space in my RAM.

Kontakt 4
PANIC: Could not allocate enough memory. Application is likely to become unstable.
