Not necessarily. You can most likely sequence songs on the card using purely software. If you decide you're having fun, you can always get a keyboard later on. You'll need a Mac serial MIDI converter, or a keyboard with a Mac/PC serial port (yes, there are some, mostly older ones)
The first thing to find of course are drivers for the card. Then you'll most likely need Apple MIDI Manager or OMS (depending on the OS and the card's compatibility) and a sequencer program. Have a search for the Shareware Music Machine for old free- and shareware sequencers for Mac.