Getting back to PSU's, I'd get as large of a PSU as you are comfortable with costwise and here's why or at least my personal take on the subject.
Contrary to popular belief, PSU's of a given wattage rating will not cram or force X amount of wattage into the system, instead all PSU's really do is give the system what it calling for to operate, nothing more.
If that were the case my Socket A's would have been nuked by my 750W PSU earlier and yes, even the 850W I have would have killed them in short order but that hasn't happened. By the same token, I would have fried the starter in an old Honda Civic I owned years ago with an 1100 CCA battery I had in it but that never happened either and I drove that car everyday for at least 3 years.
You'd want the additional wattage capacity for obvious reasons - First off a PSU that is "Just right" for your expected wattage demand wil more or less be operating at it's rated limit.
That means the unit will be loaded down at or near 100% of it's rated capacity all the time and that's not a good thing. Stressing your components at or near their limit all the time shortens their lifespan, makes them run hotter and also makes it more prone to fail and PSU's can and will take out more than themselves when they do go.
You'd want to overspec your anticipated demand for power for the following reasons:
1: The PSU when operating at or near 100% capacity isn't as capable of handling spikes in load demand, these changes in power demand occur all the time and the PSU must absorb these changes or sudden demands for it. A PSU that's overspec'ed will be more able to handle these load spikes without as much risk of ill effect towards the PSU.
2: VDroop becomes a problem if the PSU is already running at it's limit - Simply put, if it's already at it's max, you won't get any more from it and it takes x amount of wattage and voltage to achieve a given clock. Based on how hard you plan on running it, loading it down then placing even more load on the PSU always spells trouble. VDroop in itself can have a nasty effect on components under harsh conditions and will contribute to components becoming unstable and running hotter along with a shorter lifespan = More prone to crash or even fail. A good example would be trying to start a car with a weak battery vs using a good battery. A weak battery will make it turn over slower along with the starter getting really hot in a very short amount of time.
The electrical energy once introduced into the system from the battery has to go somewhere and if it doesn't have enough "Ommph" behind it (Low voltage/low amount of amperage), instead of it being dissapated as motion or work performed it's wasted as heat as in the starter itself heats up fast compared to one used with a good battery. Keep sawing away on the starter with the weak battery and the starter will fry very quickly.
Your system behaves in the same basic way, if it doesn't have enough "Ommph" behind it, it too will be subject to the same effects and that's why an overspec'ed PSU isn't a bad thing. Some will say it will use more power but that's really not true - PSU's feed the system what it needs based on demand for power, they don't cram or force excess wattage into the system and the extra capacity simply means it can handle a heavier demand for power more comfortably.
Personally I tend to go somewhere between 100 to 200W higher (Sometimes more) than my expected demand under a heavy load to be sure all is OK. This also helps to an extent for future upgrades for better components that might draw more wattage than what you are currently using.
Note as stated this is my personal take on PSU's and so far it's worked well for me. I've never had a PSU kick the bucket under any conditions (Yet).
