If you are using an HTPC strictly for music, you really don't need a fast processor, you can spend the money on hard drives.
Also, if you are using SPDIF, you don't need an expensive sound card. It simply will make no difference. Think of an SPDIF port on a sound card like a USB port on a USB expension card, you can't really get "better" quality. It's just a digital port connected directly to the computer. Any and all circuitry on the sound card is pretty much bypassed. I think the MSI motherboard you are looking at has SPDIF on it, that should suffice.
If you want to use it for TV and DVD's, you will want any video card with DVI out. If you don't plan to be gaming, an ATI Radeon 9200 PCI or AGP card will do just fine. I wouldn't recommend getting an AIW card simply because if you want to upgrade, you have to get another AIW card. Dual HDTV tuners would do the trick.
I am a huge stickler on the quality of MP3's and my digital music. As we speak I am converting all of my music to pure lossless format, which is the exact same sound quality as a CD. Trust me, even at the highest setting on an mp3, you will be able to tell that it isn't a CD, especially on your axioms.