You DO NOT want to mix music on headphones. You want some decent FLAT sounding speakers about 5 ft from each ear. If all you have is some computer speakers you dont want to use any EQ , you want them flat. And you want to mix your music kind of flat as well.. Reason being most people put their EQ in the smilely face configuration when they are listening to the music you have mixed. If you start out with the smiley face config your music will be very muddy and sound like crap. I do this for a living
If you getting into music editing/mixing dont waste your money on an x-fi or audigy, get a real card and some decent speakers. Like these for entry level http://www.sweetwater.com/store/detail/DX4/
and this would suffice for a soundcard http://www.sweetwater.com/store/detail/Audiophile2496/
But dont get the consumer grade crap x-fi and blah blah.. Bring on the flames.
ahh i didnt see your budget. , what i posted is the extreme bare minimum i would use