NETWORKING

I have a 250 user network connected to my Headquaters some 600 miles away. I use a 56K line for my internet service. I have to go throught my Headquaters where they have firewalls setup. I am lucky to get a 33K connection to the NET. A dial up will be a lot faster. I am considering using a Cable modem and have only a couple computers connect to it on the network. How can I secure my network from outside intrusion?