The power you need will largely be determined by your graphics card.
Here is a handy chart:
http://www.realhardtechx.com/index_archivos/Page362.htm
If your psu is too weak, parts may not run well, and sometimes not at all.
I have no problem overprovisioning a PSU a bit. Say by 20%.
It will allow for a stronger future graphics card upgrade.
It will run cooler and quieter.
Modern graphics cards can have high power demand spikes that need to be handled.
A PSU will only use the wattage demanded of it, regardless of it's max capability.
Not knowing anything about a particular psu, look at the warranty.
If it is for 7 to 10 years or more, you will likely get a quality unit.
DO NOT buy a cheap psu.
A cheap PSU will be made of substandard components. It may not have all safety and overload protections.
The danger is if it fails under load, it can destroy anything it is connected to.
It will deliver advertised power only at room temperatures, not at higher temperatures found when installed in a case.
The wattage will be delivered on the 3 and 5v rails, not on the 12v rails where modern parts
like the CPU and Graphics cards need it. What power is delivered may fluctuate and cause instability;
issues that are hard to diagnose.
The fan will need to spin up higher to cool it, making it noisy.
A cheap PSU can become very expensive.
Do not buy one.