yes it should, with a better set of head phones you should be able to get a noticeable difference with a sound card.
However, it has to be a pretty high end headphone/sound system to be able to discern the difference (that or you have to be pretty apt at this kind of thing). So if you're thinking of using a 50-100 dollar headset from turtle beach, sony, etc with a sound card then I would suggest you save your money, you most likely won't be able to tell the difference.
However if you have some high quality Sennheiser or another, then a sound card will definitely help. That being said, go for an Asus sound card if you're going for an inexpensive 5.1 one, I've heard bad things about the creative recon series.