A lower multilier will enable you to raise the bus frequency even higher without pushing the CPU too far. This really only helps to overclock the ram.
Depending on how high you ram can overclock, you might try finding your CPU's maximum overclock first, and then use a combination of lower multi/high HTT until your ram reaches it's peak. This will give you your optimum overclock.
If your ram cannot achieve high clock speeds then lowering your multiplier will have no affect and may hurt your performance if you have to use a memory divider.