After pkg_delete -a, exit out of X... either by rebooting or killing X.

as Root:
1) rm /etc/X11/xorg.conf
2) pkg_add -r xorg
3) portinstall nvidia-driver-7184
4) portinstall nvidia-xconfig
5) sh /usr/ports/Tools/scripts/mergebase.sh
6) X -configure
7) mv /root/xorg.conf.new /etc/X11/xorg.conf
8) nvidia-xconfig
9) pkg_add -r kde3