if a Mk1 would be running out of memory at around 40GB where does a 16MB Mk2a run out at roughly?

It doesn't work like that. What Hugo means is...

- Each song is a database entry.
- Each database entry takes up a fixed amount of space in RAM.
- The more SONGS you have on the player, the bigger the database.
- The bigger the database, the less room there is for caching the music.

So it's not a 1:1 hard-disk-to-memory ratio. For instance, just because you have a bigger hard disk doesn't mean you'll put more songs on it. Maybe you'll just encode your songs at a higher bit rate, and use up the disk space that way.