heres a thread to read about alc1150 vs a cheap soundcard like the dgx
http://www.head-fi.org/t/685163/a-researched-question-about-realtek-1150-vs-sound-card
will your motherboard support the ag1x? sure, i would not see why not. with that said, its certainly not the same as having a decent soundcard but whether or not you care enough to use a soundcard is another matter entirely.
generally using onboard is suggested unless you experience problems or are in a situation where a soundcard is required. things such as having distortion, static, etc or cases where your onboard is not powerful enough for a certain headphone choice.
the ath-ag1x is based on the ad500x / ad700x level of headphones so will have a wide soundstage, good...