For Linux, configure your NIC to use the internal interface IP address of your box that is connected to Internet as gateway IP.
So lets say your other box ( that's connected to Internet has this addresses ).
IP of ppp0 - xxx.xxx.xxx.xxx
IP of Internal interface NIC - 192.168.0.1
Then configure your Linux box NIC as
IP - 192.168.0.2
Gateway - 192.168.0.1
Netmask - 255.255.255.0
Network - 192.168.0.0
Broadcast address - 192.168.0.255