Power supplies are designed to function when the computer goes into a sleep state.
These sleep states use much less power than any computer components can even at idle.
You can never strain a power supply by using one that is rated higher than you need.
Haswell CPUs do support some new sleep states that some power supplies can't handle. It doesn't strain the power supply, but you may need to turn it off with the switch on the power supply and back on again if the system goes into these states. These can easily be disabled in the BIOS if you do not have Haswell certified power supply.
Efficiency for an 80 plus power supply is measured at 20%, 50% and 80% load. This means that the power supply does not have to meet 80% efficiency below 20% load or above 80% load. Efficiency drops off gradually below 20%, but remember that you are drawing less power anyway so it makes very little difference. If your system is drawing 30W, at 80% efficiency this draws 37.5W at the wall or at 70% efficiency it draws 42.9W at the wall.
As for noise, you have two cases. Noise at idle and noise at load.
The system in this case will draw about 30W at idle and up to 285W at load.
When picking a power supply, keep the total draw under 60% at load if you want the supply to stay quiet at load.
At idle the load will be low anyway.
Given how low these numbers are, we can cover both cases with a 500W supply.
There is nothing wrong with using a higher power supply except cost. I would however consider this a factor because you can get a much better quality supply with a lower rating for the same price.
Typically fan noise at low load will be a lower pitch, but it can be very annoying from a bad fan.
At higher load the fan noise will tend to be higher pitched.
You can also get component noise at any load.
You can get fanless models but they are very expensive and not really worth it.
Better to just get a supply with a quiet fan. A hybrid model that stops the fan at low load is a nice added extra but not really necessary if the fan is quiet anyway.
If looking at Corsair:
The RM series are their hybrid models where the fan does not run at low load. RM 450 / 550 and 650 are good. THe RM 750 and 850 are from a different manufacturer and should be avoided.
The HX series are just better quality in general and very quiet.
The Seasonic X series are hybrid models and fantastic quality, but expensive.
The EVGA Supernova G2 750 is great quality too and I haven't heard any complaints about noise, although I haven't read a review specifically on noise for this supply.
If looking for reviews on noise, look at www.silentpcreview.com.
If looking at reviews on quality, look at www.jonnyguru.com.
Most likely, if you get a top quality model 500W or higher, you won't have any problems.
This list can be useful too, stick to tier 1 or tier 2 class A if looking for a top quality model:
http://www.tomshardware.com/answers/id-1804779/power-supply-unit-tier-list.html