Try the newest beta drivers. They aren't in portage, but you can put them in your local overlay by copying the ebuild (to your local overlay) and renaming it to the correct version number.
x11-drivers/nvidia-drivers-169.07 -- The latest beta version.
I haven't had any problems with opengl implementations on my gentoo boxes. I have an 8800GT and I have a 6800GTX and neither have problems.
The relevant section of my xorg.conf (ignore the dual monitor stuff):
Code:
Section "Device"
Identifier "Nvidia 8800GT0"
Driver "nvidia"
BusID "PCI:1:0:0"
Option "NVAGP" "0"
Option "RenderAccel" "true"
Option "AllowGLXWithComposite" "true"
Option "backingstore" "true"
Option "UseEdidDpi" "false"
Option "CustomEDID" "DFP-0:/root/sedid.bin; DFP-1:/root/ctedid.bin"
Option "DPI" "96 x 96"
Option "AddARGBGLXVisuals" "true"
Option "TwinViewOrientation" "RightOf"
Option "HorizSync" "DFP-0: 30-81;DFP-1: 31.5-80"
Option "VertRefresh" "DFP-0: 56-75;DFP-1: 56-75"
Option "ConnectedMonitor" "DFP-0,DFP-1"
EndSection
Also, seeing your /var/log/Xorg.0.log would help very much in diagnosing your problems.