routers vs WAN vs T1 vs dialup

[glenn at aoi.ultranet.com (Glenn Burkhardt)]

Can I have some help with nomenclature?  I'm looking into getting a T1 line
into the office to replace our local telephone lines and dialup ISP connection.
The T1 line Internet connection is "V.35", which looks to be a high speed
HDLC serial connection (??).

We currently have a single dial up line with a static IP address assignment
on a Linux box that performs IP masquerading for all the other computers
in the office, and works as a mail server.

The guy at Net2000 said that I needed a Cisco 1720 router for the
Internet connection.  But isn't a router just a Unix box with a communications
interface that's connected to the Internet?  So isn't my existing firewall
a "router", even though it uses PPP demand dialing to connect?

It looks like I can get a $650 Riscom/N2 board from SBS or their distributor
Image Stream solutions, use the stock driver n2.c that comes with Linux
distributions, and substitute it for the PPP dialup interface, leaving 
the rest alone (with a different static IP address from Net2000).

Is it really this simple?  What have I missed?

Thanks.

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