I run GPO (for testing purposes) on a machine with 320 MB of RAM. It works fine but the simple reality is that I can\'t load a lot of instruments simultaneously, and the piano won\'t load without a warning. Keep in mind that I am far below the recommended minimum amount - personally I would recommend 1GB if that is possible for you.
You should be okay for using GPO with 512MB of RAM, which is the minimum recommended amount. 1GB is certainly prefrerable. With 512 you will find you won\'t have enough space if you try to load in a lot of instruments at the same time.