Newbie: PG8 and text file parsing

I need some advice.. I am new to PG8 - I am working on the project
that needs to load data daily to PostgreSQL from the text file.. the
text file is 40MB comma delimited file with row consisting of 20-30
fields and couple thousands of rows..

so the data needs to be parsed and if the row exists it needs to be
updated, otherwise new record is going to be created. My problem is
with parsing:

1-what would be a good approach to do it? is plpgSQL suitable for
this? or should I use PHP or other language to to that and run as a
cron job/service?

2-how about committing transactions? would there be a problem if it
is done at the end of all the updates?

Thank you
Paul