localhost tyrnov # echo "media-video/nvidia-setting ~amd64" >> /etc/portage/package.keywords
localhost tyrnov # emerge --update --ask world

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild     U ] sys-apps/portage-2.1.9.25 [2.1.9.24]
[ebuild     U ] net-misc/dhcpcd-5.2.8 [4.0.15]
[ebuild     U ] sys-devel/m4-1.4.15 [1.4.14-r1]
[ebuild     U ] net-misc/iputils-20100418-r1 [20100418]
[ebuild  NS   ] sys-kernel/gentoo-sources-2.6.36-r5 [2.6.34-r12] USE="-build -deblob -symlink" 
[ebuild     U ] x11-terms/xterm-266 [262]
[ebuild  N    ] x11-libs/libXvMC-1.0.6  USE="-static-libs" 
[ebuild  N    ] app-admin/eselect-xvmc-0.1 
[ebuild     U ] sys-apps/coreutils-8.7 [8.5]
[ebuild     U ] net-p2p/transmission-2.12 [2.04-r1] USE="-kde%" 
[ebuild     U ] sys-apps/busybox-1.17.4 [1.15.3] USE="mdev%* -ipv6%" 
[ebuild     U ] x11-drivers/nvidia-drivers-260.19.29 [195.36.31]
[blocks B     ] <media-video/nvidia-settings-256.52 ("<media-video/nvidia-settings-256.52" is blocking x11-drivers/nvidia-drivers-260.19.29)

 * Error: The above package list contains packages which cannot be
 * installed at the same time on the same system.

  (x11-drivers/nvidia-drivers-260.19.29, ebuild scheduled for merge) pulled in by
    x11-drivers/nvidia-drivers required by @selected
    x11-drivers/nvidia-drivers required by (x11-base/xorg-drivers-1.7, installed)
    x11-drivers/nvidia-drivers required by (media-video/nvidia-settings-195.36.24, installed)
    (and 1 more)


For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#blocked