DFI LP LT X48-T2R question

I don't know about the onboard. I usually have to try both and one of them will work. I think I have used HDAudio with the most success so I would try that first.

If you want to keep everything at stock (why would you on a DFI?) than you will want the 266/800 setting. If you plan on OCing than you should lower it to 400/800 until you finish OCing.

im assuming when headphones are plugged into the front case, the speakers at the back will stfu? i don't want to have to plug in my soundblaster live from 100 years ago along with the liveDrive just for plugin-headphone-turnsoff-speakers feature. it will destroy my airflow + power

Yeah, the speakers will stop. If they don't there is always a setting in your drivers to adjust that.