OK, well you've basically asked two questions there:
1) How to configure Linux as a gateway/firewall/proxy server to allow client machines access to the internet.
2) How to setup a USR hardware ISA modem.
And to answer them in that order.....
1) The most typical way to do this type of configuration is to put the modem and an ethernet card in the server machine giving the server machine connectivity to both the internet (via modem) and your internal network (via ethernet).
On the software side of things you need to be running something called ipchains as well as ipmasq. These two programs together allow for packets from your internal network to get passed on to the internet, and the otherway around. The easiest way to configure this is a neat litle program called pmfirewall (www.pmfirewall.com) which will ask you some questions about your system and then pretty much automatically configure ipchains.
2) For your USR modem, there are many out there who would be quite jealous of you having this modem. So many people these days have PCI winmodems which are a pain in the neck to configure in Linux. With your USR modem it's going to appear on a COM (serial) port of one of the following designations: /dev/ttyS0, /dev/ttyS1, /dev/ttyS2, /dev/ttS3. These 'files' in /dev/ coorepsond the com1, com2, com3, and com4 respectively. Most distributions today come with a PPP dialer that is graphical and full featured.
What distribution are you running? (i.e. RedHat, Slackware, etc.)