the refresh rate is set by the modelines which you have in your file, X will use the highest refresh rate automatically typically. it looks like ylou are running at 1600x1200 and i see there is no modeline for that resolution, i would guess we need to find a proper line for that mode.
also, i see you are using soemtying besides nv for your driver - is this the file that redhat set up? i would grab off the drivers from nvidia for your card.
btw - welcome to linuxjunior