Backup done by newbie

Hello.
I wonder if there is anyone who has administrated a "real" database that could
tell me something about backup of the database, what tools to use, how to
acctually do the backup, what backup hardware to use, and so on.

The reason for asking this is that I might have to do a backup on a database
soon, and the only idea I have right now is to dump the database and make a tar
backup of the dumped data.

The database is not that big, It's about 500k lines in the dump, that's
about <20MB of data I guess. (The real problem here is probably that it's
running on a totaly undocumented informix database from the 80's. I'll try to
switch to postgres some day...)

So, how would you guys do it? I'm quite interested in how you do it on a big
database. I mean, just to dump this database took me more than 2 hours, (the
hardware is a x86 running at 300MHz with 64MB Ram and IDE discs, so it would
probably go alot faster with some "real" hardware), but what should I do when I
get 20/2*24*7=1,7 GB of data (not to unrealistic though), i.e. it would take a
week just to dump the database? Could it be possible to just make a snapshot of
the database on weekends and during the weeks, just doing "incremental backup"
i.e. log the transactions?

I would be very happy if you would like share your experience.
Gunnar.