1. It'll play music
2. Once again, they don't really CHANGE much about the signal, so there's not much to compare
3. O2+ODAC. That iFi thing is more desktop-sh, but a bit more I believe, and it's two units (the iCan and iDAC). Your onboard is really the best though since it's free with your motherboard.
4. There's no such thing as depth in music. Sound is one-dimensional and a discrete function of time. If you want more information, there are free MIT classes, blogs, and videos on youtube. There are some good links in the FAQ.