carbon fiber fork question

I have a Specialized Allez alluminum frame and am thinking about getting a
carbon fiber fork for it. I have seen forks with cro-moly steering tubes,
alluminum steering tubes, and carbon fiber steering tubes. Are there any
differences as to strengh and durability? Also, if I decided to get a fork
with a carbon fiber steering tube, does it require a special headset (the frame
is not built for an integrated headset). Any thoughts appreciated.

Ted