What will happen when you connect the iPod depends on the settings of iTunes.
From Overview of syncing iPhone or iPod touch with your computer:
This article gives an overview of how
to set up iPhone or iPod touch to sync
with your computer, and where to get
When you connect iPhone or iPod touch
to your computer, items are
automatically synced according to your
preferences set in iTunes.
You only need to set which items are
synced in iTunes on your computer
once. After that, just connect iPhone
or iPod touch to start a sync.
Contacts, calendars, notes, and
bookmarks are synced from (or to) your
computer, so if you add, change, or
delete them on iPhone or iPod touch,
they are changed on your computer, and
vice versa. You can also sync contacts
with your Yahoo! or Google Address
Email account settings (on iPhone
only), music, movies, TV shows,
podcasts, and photos are synced
one-way, from your computer to iPhone
(though you can import photos taken
with iPhone to your computer).
If you like, you can set iPhone or iPod touch to sync with only a portion of what's on your computer. For
example, you might want to sync only a
group of contacts from your address
book, or you may want to sync only
songs from certain playlists so you
don't fill up iPhone or iPod touch
with all your music.