Config: Cisco 1720 Router Firewall + US Robotics 3453 Modem

I have a small office network. I need to determine whether I need to have a firewall configured on my T1 Cisco 1720 Router and IOS updates installed. Also, a US Robotics 3453 external modem needs to be configured for backdoor access to the Router. This is all a little bit over my head and I don't want to blow it by trying to wing it all by myself. I am fairly proficient, however, and pick up on things quickly. I've studied the manuals and did a lot of the configuration myself. I use both Windows XP and Mac OSX Leopard workstations. Also, AT&T is saying that I shouldn't have to reboot the router if their circuit goes down, and the IOS needs to be updated. The IOS is currently at 12.1(7). Thanks, James Stevens