Multiple Routes / Nat / Gateways?


I am having a problem with how to configure OS X to route through multiple gateways and was wondering if any of you guys n gals might be able to enlighten me.

We have an OS X Server with the following 4 ethernet ports

Port eth1 Ip
Port eth2 Ip
Port eth3 Ip 194.x.x.x Leased Line
Port eth4 Ip 193.x.x.x DSL Connection

Due to the different applications needed I need to route all traffic going to the gateway to the internet via eth3 and all traffic going to gateway to the internet via port 4.

I know how to do this if I just have the 1 net connection and 1 internal ip but how do I go about doing this for the two gateways?

I know the basics of ipfw and nat but not enough to work out this one..

Any help or links would be a big help