*edit In 5 minutes it took to type this I was beat to it
Any netwok settings that you will need to do should be the same as those you do during install. The things I know you should check just off the top of my head:
1. Make sure your kernel is configured for your network hardware. (module is compiled in device drivers for your ethernet card)
2. If you plan on using dhcpd make sure it is emerged.
3. configure /etc/conf.d/net
4. configure /etc/hosts
If your network card is not supported in the kernel source you have (example. gentoo-dev-sources) you'll need to do some searching in the forums or google. I use wireless and both of the modules ipw2200 and madwifi I need for my computers are in portage.
Hope that helps.
Oh and read the network part of the handbook if you have problems.