Questions about using iTunes and Airport Express to serve music throughout my house

I'm trying to confirm whether this configuration will work (reps from
my local mall's apple store weren't sure):

Background:

My house has built-in speakers in the livingroom, dining room,
kitchen, family room, master bedroom, home office, and exersize room.
We also have Cat 5 and coax wired into each room (wall jacks are wired
for phone, ethernet and cable). The Cat 5 and coax terminate in a
utility closet. We have broadband service from our cable TV provider;
our cable modem is also in the utility closet and is connected to a
Linksys router (giving us wired broadband service throughout the
house). In our home office we've installed a netgear wireless router
which my wife and I use for our everyday PCs (mine is a Dell Latitude
notebook, and my wife's is an Apple G4). We have an old G3 powerbook
with a 200GB external drive (attached through the USB port)which we've
burned all of our CDs onto (about 53GBs). This powerbook is connected
to the amplifier the powers the speakers in our home office.

Having access to our entire music collection has really changed the
way we listen to music. But right now iTunes is limited to our home
office and we've become frustrated with using CDs everywhere else in
the house. Here's what we'd like to do:

- Install a new AirPort Express with AirTunes in each of the rooms
that has built-in speakers and connect each one to the amplifier that
powers the room's speakers;
- Use our everyday PCs (anywhere in the house) to control the music
being served to each room.

Here's my questions:

1. Will that work? Will we be able to access the hard drive on the old
powerbook through our everyday PCs if we install iTunes on our
everyday PCs (including my Dell running Windows 2000)?
2. Can we serve music to more than one room at a time? How?

We appreciate all your help. Thank you.