if you are running an Asterisk release earlier than 1.4.23 then you are running zaptel. Later than that and it is DAHDI.
For zaptel, the latest Sail release is 2.2.3-x,
for DAHDI, the latest Sail release is 2.3.1-x
rpm -q asterisk
will tell you the asterisk release you have running.
There is an article on the Asterisk upgrade issues here...
http://sarkpbx.com/twiki/bin/view/Main/AsteriskDahdiUsually, to upgrade Asterisk to a new version, it is easier to simply remove the old version lock, stock and barrel before you begin.
S