they are 38ohm headphones so should be powered fine by onboard sound and do not require a powerful soundcard to drive them.
generally my suggestion is to use onboard sound unless you note audio issues (distorted sound, crackling, etc) or unless you need/want higher end sound.
would you benefit from either an external dac+amp or an internal soundcard over your onboard? very likely however i'm not sure if the gain would be worthwhile to you. something in the range of the asus xonar dx would be my recommendation if you did.
will they sound twice as good? no. you may notice some small differences here and there such as a little more bass but generally the ad700 doesnt respond much to amping...