Kernel not recognizing your hardware? Problems with power management or PCMCIA? What hardware is compatible with Gentoo? See here. (Only for kernels supported by Gentoo.)
This is a common problem with out-of-tree graphics drivers. Check that the kernel picked by the nVidia drivers build is the kernel you are running. If I am right, your nvidia.ko is incompatible with the running kernel, so the kernel refuses to load it. Xorg then fails to start because the user-side nVidia libraries cannot talk to the nVidia kernel driver.
If you still have trouble, please post the output of emerge --info ; emerge --pretend --verbose x11-drivers/nvidia-drivers.
You could also try switching to the open drivers, which keep their kernel component in-tree and avoid this problem.