By default, on a 4GB 32-bit system, windows only gives 2GB to the application. To increase the amount of total user virtual address space, edit your boot.ini and include the /3GB option. With the /USERVA you can tweak the amount between 2 and 3 gigabyte. So you can not allocate more than 3GB for applications.
Furthermore, you must relink your application with the /LARGEADDRESSAWARE option to make use of this ‘4GT’ feature. You can also modify an existing binary by using editbin.
source: MSDN