I have a late 2009 21.5" iMac and want to upgrade the graphic card. I have found someone have sucessfully done it in a 2010 21.5" iMac. Also, others have replaced HD6970M on late 09 27" iMac. Can't find anyone done with HD6770M. Just wondering, it is possible to repalce HD6770m of HD4670m in late 2009 21.5" iMac?