how can i use SELECT to find a substring of a column's data?

hello everyone i am very new to postgresql 7.3.3 and am using it on my
mac G4 OS X and i am having some trouble which i hope you can help me
out with some answers to my questions:

1.) How can I get a list of users currently logged into the database?
Is it possible?

2) I plan on using postgresql for a posting board database, how can I
search for a certain word that is in the post's topic subject header?
For example if the post subject was "Flying to the moon" and the user
wants to find that post he/she enters 'moon' in the search field I
made, I've used the : SELECT * FROM mytable WHERE (subject) LIKE
'moon'); command to find a one word exact match but don't know how to
find just one word in a string of words using postgresql. ( web/http
is not an option for my posting board. )

3.) Is having the default user 'Postgres' a security risk? wouldn't a
hacker look for that default user name and thus make it easier for him
to break in?

4.) Is there a database table limit? (can I have 700 tables in my
database?)

5.) is there a text length limit? as I said I plan on using postgresql
and your plugin for a Posting board and also an email system for my
server so I am wondering if there is a limit to the posts and email
lengths?

6.) How can I backup my postgresql database and tables? is it possible
to backup only certain tables and not the whole database?
Is it possible to use your plugin for regular scheduled backups?

7.) how many users can a mac OS X system support at one time while
maintaining decent/good performance? does postgreSQL have a limit for
mac servers? i've read that windows has a limit of only about 50
simultaneous users. i have a fast G4 and if i have to will buy a G5 i
suspect i'll need to support 200 simultaneous users that are actively
file swapping. can a G4/G5 handle that?

8.) can anyone recommend a really good book for Mac OS X and
postgreSQL? it's very hard for me to get good help/information online,
it's slow and complicated to me and it also hurts my eyes. has anyone
read O'reily's book 'practicle postgresql'? is it good?

I know these are alot of questions but for some reason i am having a
really hard time trying to figure it out on my own.
thank you very much everyone.