re: Power supplies - you're right.
It always pays to use overkill on power supplies. A 500 watt supply in a system that draws 325 watts will draw little more actual power than a 350 watt supply would, but the large margin lets the power supply be more "relaxed," for want of a better word, and it will last a lot longer.
I plugged an old system in once and one of the capacitors blew with a sound like a cherry bomb. Thought I'd have to change my pants. A close look at these capacitors reveals a "burst disk" arrangement on the end of the can, which shows that this is not an unknown phenomenon, but it sure surprised me.