Starting with the onboard sound and taking your time on the search for better sound is a good way to get going here.
If you already have an excellent sound card, by all means use it and disable the onboard sound. The onboard sound will of course use resources on the board (ram, CPU, etc) and as such will cut into performance on the board, while other operations on the board will cut into the performance of the sound chip. But since i7 is, for this generation, mostly overkill, you're probably overbuilding on RAM too (6GB anyone?) so except for the most demanding apps/games/virtual-server setups, you may not notice the resource diversion.
A cheap sound card will not sound better than the onboard sound in most cases. For one thing the cheaper ones will often use onboard resources (even the cheapest X-Fi package, like the one bundled with at least one X58 mobo, uses onboard RAM and CPU cycles). For another thing, lame as the onboard solution will be for the capacitors & cores that typically give "warmth" & "depth" & "sustain" to music, the cheap cards will be just as bad.
An expensive set of "speakers" will go a long way toward fixing this. When you're buying amplified speakers, the amps are the most important determining factor in how good your sound will be. Unfortunately it can be hard to find info on which ones have good amps in them; your best bet will be to find a hifi store or three, and see what PC speaker kits they are selling. In general though you can assume that there are not many "magic deals" out there: the best sound will come from kits that are big, heavy, and expensive. Frankly, you pay for the convenience of building the amps into the speakers - with lower quality sound and less-flexible options. We used an older sound system (5.1 speakers + 2 amps) left over after upgrading our home theater; trying a couple of the "best" integrated sets, they sounded nice, were smaller and more convenient, but did not compare to the older separates.
An expensive sound card, on the other hand, can make a big difference. I put an X-Fi Elite Pro into an Asus high-end mobo that had an excellent sound chip for the time, and the difference was huge. It took the system from "really good sound and great for games" into the realm of "home theater equipment" and pretty good for amateur studio recording as well. And while we use the optical connect for serious sound processing, for general listening we actually prefer the tonal warmth we get from the X-Fi's onboard D/A conversion. Using the mobo's onboard sound chip, that was not the case, and on your Asus board I'd expect you to get as-good-or-better sound quality with the optical connect and using the D/A conversion built into your amp.
Faced with the same dilemma, I did the following:
I connected my X58 onboard sound to my stereo receiver, then did the same with my old system with an (inexpensive) Voyetra Turtle Beach Santa Cruz sound card.
Then I played the same .wav song on both computers simultaneously.
Simply switching between the inputs on the stereo receiver I could compare the sound and found out the answer for my case: the sound card sounded "better"!
Of course it would depend on the sound card and what all you use it for but 2009 X58 on board sound couldn't beat my old 2002 sound card for music playback.