Do plan to go with the Mixtrack but what would be a decent soundcard? I've read everything from a 2.50 usb part to a Behringer UCA222 to a Traktor Audio 2?
When I used an NI soundcard, I had a hell of a time with it and never actually got it to work. I think the one I had was an audio2dj which was the precursor to the traktor audio 2. Maybe the traktor 2 would be better and wouldn't be as glitchy as it's precursor. Otherwise, I recommend the Numark DJio which they just revamped and made alot better http://www.numark.com/djio