FLAC is lossless, same as WAV. FLAC is however better in the fact that it can compress the file without losing any data, vastly reducing the size of it compared to the average WAV file.
Being able to hear the difference between 320kbps MP3 and a FLAC? HIGHLY exaggerated. It's almost impossible, even on blind tests, to make out that much difference. FLAC was designed as a storage format, so that you could convert it to other lossy formats easily, but always have the original lossless file on you. In that way, it's a library of songs, that can always be converted to the newest formats. Whereas with MP3, converting it to anything else would incur further data loss.
Basically, the difference should be almost impossible to notice. Keep FLACs for storage.