Hans, I talked about the Linux script because I guessed you would like to try it too. [;)]
But you're right, I didn't give enough details. Consider the each line starting by -D as a parameter you should add between exec "$PROGRAM_DIR"/jre1.6.0_43/bin/java and -Xmx... in SweetHome3D launch script, and separate each parameter by spaces (not line returns).
Hope you can handle it. [:)]