I swiped this information from the Boston Globe Extra Bases blog. With Manny's home run against New York yesterday, he became the fifth player to hit 50 or more home runs against the Yankees. The other four:

Jimmie Foxx (70)
Ted Williams (62)
Hank Greenberg (53)
Yaz (52)

Interesting that four of the five players who have done this spent significant parts of their careers with the Sox, isn't it?