OP,
I've looked at your specs and it looks like the on-board audio is roughly equal to the most <$100 sound cards so unless you are looking to go with digital out or have identified noise issues, or just want higher quality DAC of a high-end card I'd stick with the on-board audio.
I'd have to disagree on this one - YMMV on onboard sound card quality will vary a lot depends on specific motherboard implementation. Some done ok, and COULD be somewhat close to Asus Xonar DS, some will sound terrible.
It'll also very depend on your speakers - If they are good enough for you to even notice the difference.
Asus Xonar DS is card done right, like I said - not top of the line, but expect that extra $150 or 400% [for xonar essence] will not give 400% better sound
