Tony,
Most OEM sound cards are more than adequate for our purposes, especially when you're firing a midi file through your keyboard and using the keyboard's sound engine. The sound card has some bearing on audio recording and playback, but again, much of the audio quality will depend upon the software you're using to record with and not the sound card.
However, if you intend to play midi files directly from your PC, the sound card, and the associated software will have a significant bearing on the sound's overall quality. Of course, you will also need a relatively good speaker system to go with the better sound card or you will not gain anything.
The bottom line is there is only so much the human ear can hear, and more often than not the computer's onboard sound card will be more than adequate for most OMB needs. As for the audiences, they could care less--they just want to be entertained.
Good luck, and I hope everything is turning your way down there in sunny Cozumel.
Gary
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Travlin' Easy