The host system must have enough memory to run the host operating system, the guest operating systems that run inside the virtual machines on the host system, and the applications that run in the host and guest operating systems.
The minimum memory required on the host system is 2 GB. 4 GB and above is recommended.
To support Windows 7 Aero graphics in a virtual machine, at least 3 GB of host system memory is required. 1 GB of memory is allocated to the guest operating system and 256 MB is allocated to graphics memory.
See your guest operating system and application documentation for more information on memory requirements.