Executive summary: If you include "repo .../updates/security" in your
6.5 Kickstart config, add "xorg-x11-drivers" to your %packages
section.

...

For 6.6, Red Hat scattered the X server's driver modules into a ton of
"xorg-x11-drv-*" packages and added a master "xorg-x11-drivers" that
depends on all of them. Also, they added "xorg-x11-drivers" to the
@x11 group, causing all drivers to get installed whenever X is, in
general. This change appears idiotically gratuitous, but I am sure
they had a good reason.

Anyway, SL6.5 now includes the 6.6 X11 packages as security updates,
but 6.5 does not include xorg-x11-drivers in the definition of the
@x11 group. (Aside: Can updates override group definitions?) So, if
you include the updates/security repository in your Kickstart
configuration, X11 will fail to start because it will be unable to
find whatever module it needs for your video hardware.

A work-around is to add "xorg-X11-drivers" (or the RPMs for the
specific driver(s)  you require) to the %packages section. And then
leave a comment reminding yourself to remove it when you migrate to
6.6.

At least, this was my experience...

 - Pat