Law of diminishing returns, when I first purchased an Asus Essence ST I could notice a difference on desktop speakers, the cards such as Essence ST are better as there more honest in sound reproduction, then again if your happy with the sound from your Realtec I would stick with it.
As I side note, I don't think any sound cards are worth it (including on-board) and we are all better with quality external DAC's run from motherboard digital output, this is only for 2.0 sound of course. You get less interference and far more future proof driver wise.
I actually own 2 Essence ST's I intended to run one in a HTPC connected to quite a high end home HiFi, (Pioneer digital Amp, £800 retail of yamaha speakers) in the end I removed the Essence ST and used the built in DAC inside the Pioneer amp, the sound output was near identical however the Pioneer AMP/DAC had less interference in it. I write this to back up what I was saying about interference from sound card, and external DAC's are better.