I use THIS one when I'm planning a new build.
I always allow AT LEAST 50w on top of these calculators.
In fact, I'm gonna start adding 100w on with the power rigs can consume nowadays.
If your psu is not a good make, and cannot give a dependable and steady supply, you will have problems.
It may give you any number of problems of which crashing could be the least worrying.
Your rig is only as good as its weakest component. Save money on a cheap PSU, or value RAM, and that will be your rig's bottleneck. Every rig has a/some bottlenecks, the trick is to balance them so none stick out.