The speed the videocard operates on has nothing to do with your motherboard being locked or not. You can downclock, or overclock, your videocard independent of anything the motherboard is doing. The motherboard being locked stops you from overclocking the CPU, since to overclock the CPU you need to change settings in the motherboard bios.
Speaking of the bios, there is a slight chance that a new Nvidia card may not work with your motherboard if your motherboard does not have a UEFI bios. Does it?