For a first time Gentoo user, things can get a little hairy if you change network cards from a previous one. After you have finished adding kernel support and recompiling your kernel (unless you run stock), you might be a little frustrated to see Gentoo will recognize the device, but not use it. There's actually a quick and easy fix to this:
Get your text editor of preference up and running as Root and load the file at:
/etc/udev/rules.d/70-persistant-net.rules
You will then see the udev configuration file for your networking devices. They should look something like this:
# PCI device 0x1186:0x4b01 (skge)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:22:b0:e2:c5:86", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"
# PCI device 0x14e4:0x1653 (tg3)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:01:6c:a5:e8:be", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME=="eth1"
The entries for "rules" will be on one line each, and you can remove a device from the configuration by simply commenting it out (prepending a hash character to the line). For me, I just had to comment out the first rule. You can also change the name of your other device to the previous one, such as changing "eth1" to "eth0". All that's left now is a reboot.
See? Not too much of an effort.