MSI recommends a minimum of 550 Watts. But keep in mind the rest of your Computer Hardware needs power too.
You'll need more if you're using more high end hardware like an Overclocking CPU, Water Cooling....etc
650w is enough if you have a couple of hard drives and you're not extremely overclocking both the CPU and the GPU. However, if you have 5 HDDs and overclocking both the CPU and the GPU you're going to need more, not to mention that some heavy headphones and whatnot also use power. Just don't push things out of proportions and you should be fine.