[Solved] ASUS Xonar Essence STX - do I really need it?

I'm having a computer custom-built for me this month, and aside from the usual Internet-related stuff, I'm going to be using it for watching Blu-ray discs and listening to music (no real gaming). The computer's going in my bedroom, so I don't need a big speaker setup -- I just bought good 2.1 speakers (Swans M10) and was thinking of going with the ASUS Xonar Essence STX sound card for really good sound. However, the guy who's building the computer for me assures me that onboard audio would be more than sufficient for what I want to do. Although I can appreciate good sound (or, more importantly, I'm very annoyed by bad/unclear sound), I'm not a true audiophile, so I'm not sure I really need to drop $200 on this card. Is there generally a very noticeable difference between sound from a high-end sound card and onboard audio?

I'm also going with one of the ATI Radeon HD graphics cards (4670 or higher, not sure yet), which process audio on their own and send it through the HDMI. How would this work with the audio card -- is it a pain in the ass to even use a separate sound card when the graphics card handles sound? Would I have any kind of issues setting this up? I'm not sure how all of this works, so if anyone can explain it to me, I'd really appreciate it.

Basically, would I be wasting my money on the ASUS sound card? Thanks to anyone who can weigh in, and let me know if you need any more info about the computer itself.