The Nvidia recommendation is more 'trustworthy' as long as the power supply is tier 1,2 or 3.
(
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
)
Tier 4 and 5 cannot be counted on to have a reliable continous power output or even an accurate quote.
Nvidia state a 500 watt power supply for a gtx970.
Appreciate,this has to be balanced against a possible FX 8350 cpu which is a normally overclocked cpu,so you should be fine.Consider the gtx980 also states 500 watts.
In everything there is a little wiggle room.
If you run a clocked 8350/gtx970 with an XFX T.S. 550 unit you will likely be fine.
If you try it with a CX600 corsair unit you may come to regret it.
Common sense and an appreciation of the $ note is as good an indicator as an online calculator.
Bargains happen occasionally but expect to pay >$10 per 100 watts.
Physics is an unforgivingly constant Mistress.