my 7900GTO is now a 7900GTX

ive started on my AMD 4600+ x2 which started a 2.4ghz per core and im now at 2.7GHZ
however i can not currently go any higher, my temperature at idle is still around 19 - 21 and its not topped 26 at load yet so its pretty healthy on the temp side of things
any way i now need to downlockc my memory so that i can continue my overclock, i could not ifnd the settings in my BIOS, is their any other way to do this other than in bios?
john