FLAC lossless audio compression is 100% CD quality, but requires 300 MB per CD. I own about 1,200-1,300 CDs (after 16 years of collecting) of which I've ripped about 800 so far. By the time I'm done I'll have used at least 406 GB for them plus another 406 GB as offline backup.
Besides OS, the other space on the music server I'll be using for MP3 or AAC versions of the CDs for use with my flash MP3 player and/or an ipod.
On my desktop machine much of the space is empty so far, but just loading OS, office, Visual Studio and VS.Net has used 14 GB, and some of the games I've loaded use 1 - 4 GB apiece.