it depends on your headphones. I have that soundcard and I use it through a pair of Sennheiser HD25 SPs, which have a high impedance/ohms rating.
this means that there is more 'resistance' when the electronic signal is going through the headphone, and you can loose clarity if you're listening to a weak source (such as a small mp3 player). I found that with my headphones I could definitely tell a difference between using an ipod or phone and using my PC with soundcard (with the headphone amp enabled). this is me though, and your headphones will behave differently.
unless you are using headphones which benefit from an amp (from what I've HEARD they typically are 35+ ohms) then I'm not sure a soundcard would benefit you.
+1 to headphone amp
im only using a little cheap one but it makes a big difference