Question to safe way for minor update

Hello,

two questions regarding postgres installation:

1. i am running a large postgres 8.1.4 database in production. What is a
good and safe way for an minor update to 8.1.8 (less downdown and possible
jump back option)?
Installation in a separate directory?
--> maybe i could compile it with installation path to
/usr/local/pgsql/current , but current is an symbolic link to the actual
version 8.1.8 - and i could jump back to the old version. Any problems
here? During install i have defined the --prefix to
"/usr/local/pgsql/current -  during make install -> could i set this to
/usr/local/pgsql/8.1.8 and later set the symlink of current?

How riskful is an minor update for the data files?

2. i am using two dualcore opterons with 8GB memory - if i am using the
CFLAGS="-O2" option for compile optimization --> is there any real
performance boost?

Thanks a lot for answer,
Thomas