Each stick of RAM has what's called an SPD, which stores all of the timings and frequencies, such as 5-5-5-18, 400MHz. The reason for the 'underclock' is so that it will be sure to boot in ALL systems. Some motherboards may not like 4-4-4 timings, for example, which would leave you with a non-bootable system.
You can go into your BIOS and set everything manually though:
CL (Cas Latency) - 4
tRCD - 4
tRP - 4
tRAS - 12
Also make sure your computer has 400MHz for the RAM selected, although I'm quite sure it will be default, especially if you are using an 800FSB or 1066FSB processor. Make sure you choose the correct RAM voltage as well. At those speeds, I assume 1.9v would be more than enough.