In which file should I configure the dual-Nic routing for linux?

In which file should I configure the dual-Nic routing for linux? -- Linux general technology-Linux technology and application information. for details, refer to the following section. Centos6
Two NICs
Eth0 192.168.3.100 gateway 192.168.3.1
Eth1 192.168.7.10 gateway 192.168.7.1
192.168.3.1 is fixed
192.168.7.1 is an adsl dial-up.
Because the intranet is all 192.168.3 network segments, I want this centos to access some network segments on the Internet using the adsl dial-up line to avoid congestion.
Although the route add command is useful, it will not be available after restart. I found the method to configure a persistent and valid static route:
There are some problems with writing the route add command in the script startup method.
Then there are the route-eth0 and route-eth1 files under/etc/sysconfig/network-script/, but which file should I write? Write all?
Then there is the/etc/sysconfig/static-routes file, but I don't have this file on this centos. it is certainly easy to create a new one, but this is global effective?
What is the relationship between route-ethx and static-routes? What is the priority?