[CentOS] X/Display resolution configuration

I've installed CentOS 6.3 on a new system. I've a nagging problem that 
I'm trying to fix---the screen resolution changes. I've a flat screen 
monitor that has 1600x900 capability. However when I logout and then log 
back in the resolution changes to 1280x1024.  When I looked at the 
xorg.conf.d directory it was empty---both in /etc and in /usr/share.

Where is xorg.conf and it's monitor section now? BTW, I'm use KDE.

mw

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