A computer setup uses as much power as it needs, having a PSU that has much more power than needed just sits there, its costs no more in power terms. But there is one good thing, as electronic components age they need more power to do the same job. The main cause of this are the capacitors. I use a 5 to 10% number (per year) for cap ageing, so its always a good idea to get a bigger PSU than needed, and you can add more to the box without fear of an overload.
In the last few years the humble PSU has become something that more people look into, IMO its one of the most important building blocks, a bad PSU can break anything and/or everything else. I have seen an overloaded PSU take out the motherboard, CPU, graphics card and some hard drives with it, but also just kill a few caps.
As said most people get caught up in the watts number, the amps are a better gauge to real power, I was taught to think of it in terms of a hose pipe. The watts are how wide it is and the amps are the amount of water flowing thru it. I used to work for Micro Dynamics where they made electronic ignition systems and I cant count the number of times I got a 150,000 watt jolt from the coils, I am still here because they are rated in milli amps, 240v/13amp mains can cause much more damage.
As for PSU's I am a big Seasonic fan they make some of the very best you can get (and very quiet) and many other big names are made by them, including Corsair and PC Power and Cooling.
If you want to find out the truth about power supplies Jonny Guru's site is a good place to start, and the extreme PSU Calculator for working out how much you need to use for a basic watts guide, you can then work out the amps you need on 12 volt line (most important) then 5 and 3.