[CentOS] CentOS 5.3 - changing display settings

I've just installed CentOS 5.3 on my desktop and need to change the
installed display setting from 1280x1024 to 1024x768 (more suitable for
my monitor). When I try this on the Display Settings tab, I get the
message:

"Display settings changed
You need to log out and restart the X server for the changes to take
effect.
Configuration was written to /etc/X11/xorg.conf, original configuration
saved as /etc/X11/xorg.conf.backup."

Trouble is, I don't know how to restart the X server apart from
rebooting the computer, but that has no effect and the display settings
remain unchanged! Any help appreciated,

Andy