Ok I finally found a way to install more than 2GB RAM to my iMac
the solution is the modify the firmware of the logicboard and make him believe that it is iMac 5,1 (late 2006 core 2 duo) instead of iMac 4,1 (early 2006 core duo)
the is a download available here: [http://forum.netkas.org/index.php/topic,1122.0.html|iMac firmware upgrade]
but to do this one before should upgrade the CPU of the iMac with a core 2 duo CPU (the ones from iMac 5,1).
I tested it and works, this way my iMac has a maximum 3,3GB RAM :)