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.xx
• TNT, TNT2, Geforce and Geforce 2: max 71.xx

In order to properly install the correct version, it is recommended to mask the packages like so:

For Geforce 3 and 4

Add the following to /etc/portage/package.mask:

>=x11-drivers/nvidia-drivers-1.0.9700

For TNT, TNT2, Geforce and Geforce 2

Add the following to /etc/portage/package.mask:

>=x11-drivers/nvidia-drivers-1.0.8700

This should ensure the correct version will be merged, and you won't be bothered each time to install a new version.

For more information about this, I strongly suggest you also read the page at the link above. It tells you more about installing and using the driver.