Good questions!
If you consistently get 100 FPS or more, a high speed monitor will give you a more fluent motion experience as 100>60. Also, you don't have to limit yourself to a 120 Hz monitor, they all can be limited to a lower refresh rate. If you want a high speed monitor now because of a good bargain or because you just need a new monitor, that's fine. Just set the monitor speed according to your system. If you plan to get a more powerful video card sometime in the future you can then unleash the full power. That might feel like a whole new rig, wouldn't it?