If you already have music in the mp3 format, converting it into the FLAC format will not improve the sound. The important factor to remember is that the FLAC and Apple Lossless formats are lossless formats, they provide bit perfect copies of the original version. So if you start with a good original then when you convert it to FLAC or Apple Lossless you will receive a good copy. If you start with a bad original (mp3), then you will end up with a bad copy (since it is a bit perfect copy).
If you want to use iTunes, look at converting your music to the Apple Lossless format instead of FLAC. Since they are both lossless formats, they will both provide perfect copies. But you will be able to listen to the Apple Lossless music in iTunes. If you are concerned with the size of your music files and the Apple Lossless files are too large for you, then research the ALAC format.