linux firewall for NT domain

I've got a client that has an small NT domain (2 servers, 5 workstations).
They have no broadband access and two of the boxes use modems to dial-up to
an ISP. This is a bad thing because their database server that houses an
OLTP system is quite exposed when they dial-up. (ah, the joys of NT). I'm
considering using an old PII 300 with 64 RAM running rh as a firewall for
this ghastly situation (cost is a major factor, for one thing). They are
going to be migrating to a T1 in the future as well (something to keep in
mind).

My question(s) is(are):

Are there any docs specifically on linux firewalls/nt domain/samba/modems?
I've read the Firewall How-to etc.

I haven't dealt much with NT/linux on the same LAN. I'd say I am an
"intermediate" linux user, but mostly development. (haven't done much
firewall / ip chaining stuff).

Any pointers would be great.

thanks,

todd