In normal usage (playing its internal tunes and your own MIDI files) the HT-300's standard memory card will prove more than sufficient. However, if you want to store a large number of MP3 files you may need a bigger capacity card. Compatible cards are available from Hymn Technology Ltd, correctly pre-formatted for use with the HT-300.
Some generic compact flash cards are NOT compatible with the HT-300 and could possibly cause a malfunction.