First things first, I'm looking at the specs of that series switch, and I'm wondering if your "T1" line is linked to another switch? It may be a FDDI ring you're referring to. It has ATM capabilities if you have the module installed. Is this the case?
In my opinion, you'll need a router if this is a standalone office. If it's a part of an existing LAN, then you'll need to find your gateway to configure your machines. This may be very superficial to your question, but you didn't really specify a task you are trying to accomplish besides internet. Your Geographic Information System will automatically be available on your LAN, if you have not acquired a switch which is already configured with static VLANS. Hope this helps.
Bruce Davis, CCNA since 2003