Weird, my monitor is 4:3 and I can select 1280x720. It's possible that, for some reason, the resolutions your monitors supports is not being detected correctly by your graphics card driver. If that's the case, it will stop you selecting resolutions and refresh rates it thinks your monitor does not support. I don't know about nVidia drivers, but with ATI at least you can turn off EDID in the CCC and specify your own maximums; you should then be able to use any resolution lower than that.
I think nVidia drivers also have an option somewhere to add custom resolutions; that might be a more convenient solution.