Jesus....!
You won't notice any improvement with a dedicated sound card. It might sound a tad "different" at first, but after 2 minutes you'll be tuned into what ever difference it has brought, and won't notice it again.
For me, I'd only buy a dedicated sound card for gaming performance reasons. If I was buying one purely for sound, I'd buy an external one anyway - the less audio near the various parts of the computer the better imo.