[gentoo-user] Weird GNOME problem

Earlier tonight I changed the setting in /etc/conf.d/rc for the
RC_DEVICE_TARBALL from "yes" to "no" and rebooted.  Now I can't get into
GNOME.  For my personal account after I enter my username/password the
screen clears and I see the default background color and my mouse
pointer, then nothing else.  I can move the mouse pointer, but there's
nothing the click on and the context menu doesn't come up when I
right-click.  When I tried to log in as root from the welcome screen I
see the "GNOME Starting" box with the Gentoo logo, but it never goes on
from that.  To get out of both my personal account and the root account
I had to Alt+Cntrl+Backspace.  I got a Failsafe Terminal and looked at
the log files for gdm and saw this:

(EE) GARTInit: Unable to open /dev/agpgart (No such file or directory)

There are five log files in /var/log/gdm; they all say roughly the same
thing, and their timestamps are consistent with the times I tried to log
in and got stuck.  I can't even find out why it's looking for this file
or how to make it stop looking for it.  Can anyone help me out here?
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