RHEL5 custom screen resolution

Hi, IM trying to create a custom single screen resolution. I have found solutions that use xrandr that allow you to generate a modeline which can the be added using xrandr --add-mode but RHEL5 xrandr doesn't seem to have the option to add modeline which can then be referenced in the xorg. I have tried just plcing the resolution in the xorg but as the grpahics card doesn't natively support it just goes to one of the ones it does support rather than the custom one. if I set just the custom one then X crashes out.

Is there any tips anyone can offer me to get a custom resolution set?

Thanks