Information about nvidia in Gentoo can be found here.

http://www.gentoo.org/doc/en/nvidia-guide.xml

It goes more in detail about which driver versions are suitable for which nvidia cards; quite handy, since they kicked out nvidia-legacy-drivers.

If you don't want to read the article, here's the short story about the driver version:

Geforce FX, 6, 7 and 8: use the newest driver (100.xx and up)Geforce 3 and 4: max 96.xxTNT, TNT2, Geforce and Geforce 2: max 71.xxIn order to properly install the correct version, it is recommended to mask the packages like so:

Add the following to /etc/portage  ...