madmatt30 :
The issue with a 500w PSU is I honestly can't think of a straight 500w model that is actually really decent quality.
If I were spending between $500-800 on a new GPU I'd probably be thinking about getting a nice quality gold rated 550-650w at the same time
Matt point out a good fact XD, i did mention earlier that if ur PSU is decent quality it should be enough, however, as far as i know, most of the great quality PSU had a certain characteristic to have extra 50 on their wattage XD [unless it's the 4-digit wattage PSUs] like 450w, 550w, 650w, 750w
from technical perspective, the whole system can be sustained with a 500w PSU [since a system with 1080 tend to consume somewhere around 350ish, and the 1080ti system is around 400ish], but it might not be safe to do so [a low / mediocre quality unit might have some lacking in safety measure / output not as good as it's rating], so i recommend if u're going all out with the GPU, do consider upgrading the PSU to something much more convincing
if u had extra money and u want better efficiency, getting the 650 / 750 will yield better efficiency output from the PSU