Hi John,
My midi files, one song, 3~4 min have max. 100 Kb -> 10 songs to 1 MB.
My S970 have 13 MB keyboard user memory (if I don't make a mistake for S770 is the same, you must check)
This is not connected with Expansion memory 160MB in your case.
So look on the size of files on the computer and normally you can copy around of 130 songs.
When will be full you will have a message <memory full> and you can erase some files.
So use the user keyboard for live.
Regards
Mihai