use a memory divider and a lower hypertransport multiplier.
one will be like "200mhz, 166mhz, 133mhz, 100mhz" for the memory divider, you'll probably want to change it to 166mhz... and as for the hypertransport multiplier its probably something like "5x, 4x, 3x, 2x, 1x" although i think some offer like 3.5x or 2.5x which is weird but... that'll help you find it in the bios.
make sure you watch your heat and take all other alternatives before raising voltage!