system-config-network

You can also edit the following files and 'service network restart'.
/etc/sysconfig/network
/etc/sysconfig/network-scripts/ifcfg-eth0
/etc/resolv.conf

The syntax of what you want to do with ifconfig will only stay in effect until the machine is restarted but the commands are

To change eth0 to 192.168.1.20 with a 24 bit (25.255.255.0) netmask and a gateway of 192.168.1.1

ifconfig eth0 192.168.1.20 netmask 255.255.255.0 up
route add -net 192.168.1.0 netmask 255.255.255.0 eth0
route add default gw 192.168.1.1

(The up at the end of the first line and the route add -net line aren't always necessary, but shouldn't hurt anything.)