http://support.microsoft.com/kb/2303409

When you start Zoo Tycoon 2, you receive an error message:

You need at least 256 MB of system memory to run Zoo Tycoon 2. (___ MB detected).

This error may occur if you try to run the game on a 64-bit operating system with more than 4GB of RAM (Random Access Memory).

To resolve this issue, upgrade your version of the game by purchasing and installing expansion packs such as Zoo Tycoon 2: African Adventure or Zoo Tycoon 2: Extinct Animals.