It really depends. Some people buy a computer, use it, and then buy another one. Others upgrade at semi steady intervals. Others upgrade whenever they get some money, but never know when that is. What you should buy depends on what your answer is. If you just buy a new one every X years, then buying bigger then you need is pointless. If you upgrade semi steady, I could see buying larger then you need IF you think you'll be upgrading. There are plenty of good sub 700W PSUs, I'm using one myself. A 450-500W gold rated PSU can now be had for $50-$65USD. Not sure about NZ, but I'd rather have a good 450W gold PSU, then a 750W bronze unit I'm not even going to load 50%.