Re: How to delete a specific refresh rate from xrandr?
Thanks thecapsaicinkid, this is valuable information.
Though, as I said, the monitor was at fault: it doesn't display interlaced frames and the driver used '60 Hertz Double Scan' as the "default frequency" for 640x480. I just copied the "metamodes 640x480_75 +0+0" or something from nvidia-settings into xorg.conf and it works.
Thanks for all the info though, it is valuable nonetheless.