You could buy the card you would use in the new build and run that now as well. If you plan to keep a sound card in as a spare case then go with a low cost model that still sees some updates for Vista as well. That would be the Audigy 4 Pro in mind there while the Audigy SE 7.1 models can be found cheap at this time.
http://www.newegg.com/Product/Produc...82E16829102012