There are 3 songs that play during the anima sequence.
Isaac Hayes - Run Fay Run (when O-Ren assasinates the president)
Luis Bacalov - Grand Duel, the 1 that plays most of the time, probably this is what your looking for.
Armondo Trovajoli - The Long Day of Vengence (plays when O-Rens father gets the sword through her head]
Hope this comes to some help