Do I need a sound card with my MOBO or is onboard good enough, paired with Sennheiser Game One

comparing it to headphones its far from 'top of the line' but for a headset, its one one of the better ones. the game one is based on the hd 518/hd558 designs ($80-100 headphone).

the hd518/558 are easy to drive so will work with most products fine.

your motherboard onboard should be at least of decent quality so should work fine. honestly it is hard to judge on specs alone as even motherboards with the same audio chipset can act differently. as long as the volume levels are sufficient and you are getting no distortion, buzzing or other issues i would say onboard is probably fine. you can always upgrade to a soundcard later if you feel it will help.

if you need/want virtual surround sound and your motherboard doesnt have it then upgrading to a soundcard or using the (free, but not very good) razer virtual surround software can fix that.