setting up auto SGA

 I understand the steps to setup auto SGA. Right now, the database I have does not have spfile setup. If I modify the init.ora, to set up sga_target and sga_max_size and then comment out some parameter. That will require me to bounce the database. If I then create the spfile after the parameter modification, that will require me to bounce the database again in order for the spfile to take effect. My question is , can I create the spfile first and bounce just once for the file to take effect and then dynamically set up the sga_target and sga_max_size