I had that, and I can solve it
I just had that problem the other day. I just decided to finally make the switch. I spent countless hours online, then I emerged nirc and went into the chat. Luckilly emorphix was in #gentoo. He helped me out, And i had to learn a little bit more to (I screwed a few things up cuz it wasnt step by step) so now i will pass the knowledge onto you.
Heres my config. I have an nvidia GeforceFX5200 Agp, with 256MB VRAM. Ok. KDE 3.4, kernel 2.6.
commands to type to update your kernel as well as your xorg:
first remove the old stuff
su - // I would assume your root already if your updating like this, but for good measure
rmmod nvidia // remove nvidia from memory
emerge unmerge nvidia-glx // get rid of the current
emerge unmerge nivia-kernel //see above
lsmod // this makes sure that the module is gone from memory
ACCEPT_KEYWORDS="~x86" emerge nvidia-glx // this is so u get the masked packages
it should give u a message about running something like eselect opengl set nvida or something, RUN IT.
then run modules-update
startx and it will work.