Getting low on RAM

I have a two year old Mac Pro with 8 core and everything.
I am creating a piece which involves the use of multiple VST-instruments, such as some of the East West libraries. The piece probably have 60-70 active tracks, and i keep getting a message that says that my memory is low, and after getting this message cubase shuts down. My computer have 16 gigs of RAM, so from my understanding i should be able to run it with ease.

Is there anything i could do, or do i have to buy some more RAM?
Error message:

Which version of Cubase? You have to be running the 64 bit version to access all of your ram. If you are using 32 bit, Jbridge would help you access more. Other than that, I would suggest you freeze some vst tracks to free up more ram.

I am running Cubase Essential 5

It is my understanding that CB Essentials only offers a 32 bit version. You have to update to CB Studio 5 or CB 5 to aquire a native 64 bit version. You need to run a 64 bit version of Cubase to access all your ram. That is of course if you are running a 64 bit OS. Otherwise, you are limited to accessing 3 or less G of ram and that includes what CB is going to use. As I mentioned earlier, Jbridge may help you access more ram as it will allow you to access 2 G of ram per application or vst instrument. Adding more ram will not solve your problem.