Nvidia backports support for new X and kernels for older video cards (Riva TNT's etc) well with their "legacy driver" concept. Latest version of nvidia drivers as time of this writing is 8762, while latest legacy version is 7182 (updated quite recently). The cards which are supported only by the legacy driver is at http://www.nvidia.com/object/IO_32667.html - there are TNTs and older Geforces. Anyway, a suggestion to Xorg-server modular ebuild: Add a nvidia_legacy VIDEO_CARD that would depend on nvidia-glx <= 7999 (I'm assuming that the 8xxx and beyond are where the development continues and 7xxx series remains for the legacy). Leave nvidia as-is.
That's why x11-drivers/nvidia-legacy-drivers was added.
Ok, I assume you'll add them to VIDEO_CARDS as separate option when the binary drivers start working again with the new Xorg ABI. Cool.