CentOS - Apr 2011 - update fuzzed display -- how to probe and force driver updates

Did a yum update on a CentOS 5.5 box at home when I got back from a
business trip.  Update seemed to run without errors, but after a reboot,
the display is grainy and the colors are not true.

Have spent nearly all my time over the last couple of years working at
the network and virtualization layers on servers and have mostly ignored
the X subsystem running at home. So I'm not current with it or the tools
to configure it.

How do I force kudzu to detect the hardware and force a re-installation
of the proper drivers?

Dave M