G'day,
For good instrument sounds you need a card which stores the voices in a WAVETABLE. These store samples of real instruments (although they don't always sound like real musicians playing them). Roland Sound Canvas is a good example. Some cards which don't have this feature built in have a connector for a daughter card.

Your card probably uses FM synthesis which was a nice try when it first appeared but not worth having these days.

Cheers,
Alan