here is the signature for the
DDR2 HD4650, from the Gigabyte GV-R465OC-1GI. i used it to increase the limit on my Powercolor AX4650-1GBD2 from 650-450 to 750-550.
hint: gigabyte posts their vga bioses online, so search for an OCed version of your card over there and capture the signature from it..
http://www.gigabyte.com.tw/Support/VGA/BIOS_List.aspx