Protected Internal Network (secondary)

I have a small (CAT5) network. Except for one XP machine, all the machines
are Vista (Ultimate & Premium).

I'd like to add a 'server' to the network and will probably run XP on it.
This server will hold mp3s, images and DVD videos. I will have no need to
access this 'server' from anywhere other than my office (at this time or in
the foreseeable future), so I won't need to access the internet from this
machine, nor will I need to access it from a location outside the office.
IOW, I want this 'server' to be accessible only to the local machines on my
network.

All that being said, My question is this:

If I add a 2nd NIC only to the machines I wish to allow access to this
'server' along with a network switch (all CAT5), will this provide the
isolation that I want, or will other precautions be necessary (i.e., instead
of a network switch, will a firewall router such as the Linksys BEFSR41).

Will the 2nd network shares be accessible to the machines on the primary
network (i.e., have only one NIC and only connected to the first network)?
The outside world?

Thanks for any and all info,
James