The memory will only run as fast as the front side bus will allow it to, in most cases this is 166mhz x 2 = 333Mhz for PC2700DDR Ram.
If you already have PC3200 DDR Ram (which is what is needed for 400Mhz FSB) then your mobo must support 200Mhz FSB (or 200 x 2 = 400). If not then the only way is to try "overclocking" the FSB in the bios.
For example, I run an Athlon XP3000 that has a stated FSB of 400. My memory is PC3500 and to get it to run at its maximum I have "tweaked" the FSB to 218Mhz for a total of 438Mhz FSB. My XP300 is now also reported as a Athlon XP3200.