The tool
xAct makes it possible to use FLACs in iTunes.
I never tried it though (only use the Mac for work).
I recommend FLAC myself but Apple Losless is natively supported by iTunes. It is another format with non-destructive compression like FLAC. There have been a lot of arguments in audiophile-forums which one is the better. I prefer FLAC because it uses the same headers (metadata) as the open (Ogg)Vorbis format.
I'll post a detailed performance comparison on audio-file formats in DJ software on my blog soon.
For tag-editting on OSX
Tagalicious is very good.
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