I keep a moderate amount of music on my laptop, for ready entertainment and ambience. I know people that keep gigs and gigs of music around; I rarely get above one gig, which is good on this smallish circa-1999 laptop. In fact, I often delete most of the music directory, and replace it with things from CDs I choose more or less at random. Something about shuffle-play over a small collection seems to work nicer for me than over a massive collection.
I note that I always have a good amount of mostly-calm instrumental music (filed under "yin") and a slightly greater amount of everything else (filed under "yang" -- I attempt no other classification than that semi-hemi-dualism). And over the months, about the only constants that I note, have been this: Yin always has Brian Eno's Music For Airports, at least track 1; and Yang always has Fluke's Risotto.
An ogg of John Cage's 4'33" also tends to stick around a lot too, especially because it's all of 7K long.
It is interesting to consider that the next hard drive I get will probably be able to easily hold, at once, a rip of every CD I've ever owned.