Since when are there terabtye USB flash drives? I'm sure they're real cheap ...
And if every CD has about 12 song on it, and a song of 3:30 in length in Mp4 format, it's about 3.5-4mb in size. So that's about 50mb for one CD.
I'd say 100mb would only be CD's with 20 songs of songs that average out at 3:30. If you have about 12 songs of 4.5-5mb in size, it's probably going to figure out around 75mb. So ... in theory then, if you have 600 CD's, times the average maximum amount of space of music on a CD (100mb), that's 60000 = ~ 55GB. So I'd think the iPod Video 60GB would work (it's actually about 57GB, not 60GB).
Or you could just get a 100HD external HD like others have suggested.
And BTW, when your "storing" files for long-term use, it's called a Hard Drive (HD), not memory. Computers use memory to retain files for short amounts of time when running or opening files or applications.