Find a card on the Audigy2 or ZS line of sound blaster. Most of their cards are under 100. If you want the front bay its going to cost more. Look into sound blaster, there is nothing better than them.
If the user is listening to MP3's over a cheap pair of speakers, they could probably get away with the onboard audio. If they're doing some gaming, a hot audio card will make all the difference. (Less stress on the CPU = better performance...)
hmm actually...i have cheap speakers (Z-640) but i just got the Audigy 2 ZS, and it makes an amazing difference in my music and games too. But I listen to music more, and it's awesome
sound cards do make a difference, i have the audigy2 platinum, and just for kicks i tried my logitech 5.1 speakers with the onboard sound, and it did not sound good at all. I would recomend a sound card.