Installing RH Linux firewall in Home office for security, how can I VPN passthrough?

I wanted to install RH Linux firewall (2 NIC cards on Linux PC, one
NIC to Router and other NIC to Cable Modem) between Cable modem and
router, Is it possible to pass through all VPN packets through
firewall?. since I wanted to logon to some other office through VPN?.
Can RH Linux firewall act as real firewall box like WatchGuard
(commercially available in market)?. Do I have to install anything
extra?. The below picture shows, this way I wanted to connect it. To
make my Linux box like commercial firewall do I need to install
anything else?. Thanks.




|------PC1
----------- ----------------- -------
|------PC2
Internet---|Cable modem|----|Linx
PC(firewall)|-----|Router|---|------PC3
----------- ----------------- --------
|------PC4