I use Hoster and Windows Media player side by side with no ill effects. soon Hoster is supposed to support WMA files, and we'll be able to load up tracks in Hoster (I think) and just do it all from one program. I'm not sure how hoster will interface with the WMA data base if at all, but back to your question, the two programs play well together on my system.
As for the sound card thing, you can get away with it on a normal sound card by just minumizing the program not in use. You can toggle between the two app's without any sound loss that I have noticed. hope this helps.