I heard that you can use new voice banks via smart media cards, but in the s80 manual I read that only one voice bank can be stored on a card. So, is it true that I need a seperate smartmedia card for every new voice bank? So I also can't use new voice banks at the same time?

The Smart Media card holds the data for the external voice bank. You can save as many voice banks (S2V files) as memory will allow on the card.