Hi Alan,
For that amount of songs to be transferred, I would say it is essential to have a cross-check system i.e. name AND number in case there are repeat song titles. Anyway it is quicker to load by number from the SD Card to the Keyboard.
I wonder also if you realise that however large your SD Card is, you can only save 99 folders with 20 songs to each folder. This is due to Technics allocation of a very small amount of storage space on the keyboard and not the fault of the SD Card, so I would say you will need quite a large SD Card(s) to store all that data. I use 1GB cards simply because my camera also uses SD Cards which means I can 'mix and match' but from what other members have said from time to time, 256MB or 512MB Cards should be o.k.
I was interested in Bob's reply to you. I didn't know about the extension. You learn something every day on this wonderful site.
Long may it reign.
Best of luck!
Audrey