Onboard vs Soundcard

[Shure]

Can anyone put this into perspective for me? I've NEVER heard music through a dedicated soundcard and I am trying to understand how much of a difference it makes compared to onboard sound(Realtek ALC888)
I'm looking at the X-Fi Xtreme Music are there any better cards ~£50

I only use headphones on my pc if that meakes a difference

 

[Shure]

Could be my dumb question for the day but I'm looking at the 2496 and I dont see how I could possibly hook my headphones up to that card? am I missing something?...

Does the Revolution 5.1 rate as highly as the 2496?