The first leaked Android 4.0.4 ROM for Galaxy S II is out there, so if you are ready to flash it on your device then backup first as it will wipe out all of your data.
Some information about Android 4.0.4 leaked ROM: PDA I9100XXLQ5, MODEM: I9100XXLQ5, CSC: I9100OXALQ5, build date: June 28, 2012.
If you do not want to install Android 4.0.4 leaked ROM on Galaxy S II then wait few months for its official release.
Download Android 4.0.4 for Galaxy S II