Even though Pauly1 already made his mind, I want to tell you my experience with mp3 files.
What really counts (for me) is the quality of the recording, I have 128 kbps songs that have almost the same sound quality of others of 320 kbps, for example I have a Yuksek song ("Tonight" I think) with 128 kbps that sounds as good as Bloodhound gang´s MOPE at 320 kbps, in my case most 256 kbps songs are good enough for me, many 128 kbps songs don´t have good sound quality, I just don´t listen to them, painful to my ears
In the 256 kbps songs, I have some nice recordings that can match or even sound better than other 320 kbps songs.
Never heard a FLAC files.
The difference that I can hear between 320 kbps and CD is that CD has more bass and nothing else.
That´s just my opinion