Saving power and money is actually the last factor I cared about when recommending a proper wattage and high rated (Gold, but you can go higher!) PSU.
Higher power efficiency rating = less heat production = quieter running fans (if any)
Look at mine, the SS-460FL is a fanless 460W Gold PSU, which is in that sweet spot where its heat production isn't even high enough to have a fan installed at all. Seasonic's current version of fanless PSUs qualifies for Platinum efficiency which allowed them to introduce a 520W fanless model too. I love me that silence, sir.
Even if you don't care about noise, I'd still go with high rated PSUs because of the higher quality components they'd have to use to meet those ratings. But whatever, PSUs of any rating should all run great for the duration of the PC's lifetime.
Now back to power savings. You don't need to get an 80 PLUS GOLD rated PSU vs a regular (more commonly Bronze now) 80 PLUS but the wattage aspect still applies. Don't overcompensate.
From: http://en.wikipedia.org/wiki/80_Plus#Technical_overviewTo qualify for 80 Plus, a power supply must achieve at least 80% efficiency at three specified loads (20%, 50% and 100% of maximum rated power). However, 80 Plus supplies may still be less than 80% efficient at lower loads. For instance, an 80 Plus, 520 watt supply could still be 70% or less efficient at 60 watts (a typical idle power for a desktop computer).[9] Thus it is still important to select a supply with capacity appropriate to the device being powered.
If you're going to compare a Gold against your $70 750W 80 PLUS regular (apples to oranges...), make it more fair and look at 80 PLUS Bronze (regular 80 PLUS are rare now) options instead. A 450W Bronze is probably cheaper than a 700W Bronze, making it both the cheaper and smarter choice.
Your choice in power efficiency is flexible, but overcompensating in wattage is stupid no matter what.
