Wattage is only a small part of the equation when it comes to efficiency and noise.
For efficiency, what matters most is the unit's 80Plus rating and its efficiency below 20% load (which is the minimum load that 80Plus tests, at least until you get to 80Plus Titanium). You can optimize efficiency for your particular setup by ensuring that your idle watts aren't too much below the 20% load because typically efficiency plummets drastically below that load. A bronze unit that does 85% at 20% load might only do 75% at 10% load. Secondarily you can also ensure your load watts aren't too far above 50% load, but the importance of this is mostly theoretical. In practice it's not going to matter for efficiency whether you're at 50% or 70% load; what matters more cost-wise is not buying an oversized unit.
You'll also want to consider whether it's worth it to pay the premium for a higher efficiency unit in the first place, that is, will the electricity you save pay back the initial difference in cost in a reasonable amount of time. For a setup only using a couple hundred watts at load, about $10-15 for a Gold unit over a Bronze unit is still quite reasonable. Beyond that, you'll need other justifications for the more expensive unit, such as lower noise, better resale value, higher quality, modular cables, longer warranty etc.
As for dBa, what matters most is the quality of the fan and the fan profile or fan curve. Some 650W Gold units run noisier than other 550W Bronze units, for instance - so although rated wattage and efficiency do correlate with how noisy a unit's going to be at a given load, they're only part of the equation.
To sum up, yes 550W would be enough for GTX 970 with a stock i5. You'll be using about 250 to 300 watts while gaming, probably closer to 250. Most 550W units will not have so aggressive of a fan curve as to become noisy below 300 watts.
620-650W makes sense if you want to future proof your PC so that it'll handle more power hungry video cards or other hardware without becoming too noisy.