how do i clock down my memory speed via the bios?

hi,

i am trying to put two 1gb pc-3200-400mhz dimms into an asus a7v600 mobo.

problem is it won't boot with both in.

after doing some exploring on a couple of forums, i think the answer may be to clock down my memory to 333mhz via bios settings.

but looking at my bios all i see settings like: sdram cas latency, sdram ras to cas delay, sdram ras precharge delay, sdram active precharge delay, and sdram bank interleave.

can anyone step me through what those settings should be to lower my ram dimm's clock speed from 400 to 333?

on a related note, should i bother...i mean, will there be a huge, noticeable performance hit if i go from one dimm at 400 to two at 333? my rig is primarily used for gaming...

thanks!

steve