First CentOS 6.3 is Unsupported
the ONLY supported version in the 6 series is the current 6.4
it is advisable that you upgrade ASAP
yum --releserver=6.4 upgrade
the Quadro 4000 uses the almost legacy driver 304.88
also centos6 shipped with the Nouveau driver and NOT the nvidia.run ( or the kmod-nvidia )
PS. the kmod-nvidia for the unsupported 6.3 is VERY OLD and dose NOT have any current bug fixes
are you using the nouveau driver ?
or did you blacklist it and install the OLD kmod-nvidia using yum ?
or the current 304.88.run for that almost legacy card ?
if you are using the nvidia driver then there should be installed the nvidia config tool
you can set the resolution there
--- you need to be root to save the xorg.conf ! ---
--- your root password when asked for ----