Not all RAM can necessarily do tighter timings at lower speeds. I remember Masky telling me his Hyper-X could not. P4s benefit from bandwidth, not timings.
But yes, get the DDR400 since the price diff between DDR400 and 266 is very low. Will come in handy if you migrate to a newer system as well. Even if the mem is rated for higher speeds and has to run at DDR266, it can always downclock by itself.