It is very difficult to say as it depends on the quality of the rip. I had a quic look through my library and at 320Kbps you are looking at 10-12MB average per song. At 160 - 200Kbps it about 5-8MB.. I think. Also depneds on the songs. An epic King Crimson track will probably be slightly longer than Britny's latest offering....
You lazy sod! hehe 10MB X 350 songs = 3500MB ~ 3.5GB iPod Nano 4GB? I think they retail at less than £200.