is corsair cx650w (2015edt.) enough? or should i really buy a new 1?
PCPartPicker part list / Price breakdown by merchant
Can you confirm that this is your PSU ? if it is your fine.
my guess is yes.
Power supply calculators nearly always over estimate.
Even more of a reason the EVGA one would over estimate is because they get more profit from a larger PSU bought.
The best way is just to find a professional review of the gpu in play, with a similar CPU (main power consumers. Rest of build won't vary much). Look at the wall draw and multiply by the PSU efficiency.
For example, this review/benchmarks look at the GTX 1080ti with a 7700k. At a gaming load, it draws 391w from the wall. Since we do not know the PSU in this case, I'll just assume it's efficient to .90. 391 x .90 = 352.
Even a quality 450w PSU could get the job done.
OP, that cx650m (2015) PSU is absolutely capable.