The Yamaha QY100 uses SmartMedia cards for midi playback (it's an arranger module too). Also Tascam's PocketStudio 5 uses Compact Flash cards for midi and mp3 playback and has a built-in GM synth. However if you really want to get small, I'd just take the outputs of your PSR and record MP3 files with it, then use any of the new micro-MP3's player on the market for playback.