I've done several comparisons between onboard sound and discrete. I'm using a sound blaster ZxR, and if you're using quality speakers or GOOD audiophile quality headphones, there is an immense difference. Discrete sounds better, if you get a good soundcard.
I know people will argue about this until they're blue in the face. People are stubborn about this - some people are so stubborn about their opinions with regard to soundcards sometimes, that you'd think it's akin to a political debate....people get really silly about it. My first suggestion is don't listen to a single stubborn person in this thread - compare yourself. I suggest doing a direct comparison and ensure that you have audiophile quality gear - Buy from a place with a good return policy such as amazon and do the comparison yourself. This is what i've done - There is a big big difference - the ZxR sounds so much better than onboard audio that it isn't even funny. It is THAT MUCH better - but you MUST have a good set of speakers or audiophile headphones for the difference to be realized.
If you're using garbage speakers, then garbage in garbage out. You can use some 1998 cirrus logic garbage soundcard and be none the wiser. It's all about the complete package, not just the soundcard - a top quality soundcard will amplify the quality of your high quality surround speakers or audiophile headphones. If you're using garbage speakers/headphones though, don't bother.