Just look up a splitter for the headset problem, yet I do not recommend to use that mic for your computer. You could buy a cheap mic that you place on your desk and it would be a major improvement. As for the mobo I am assuming you have an atx mobo and the case only fits mini atx or vice versa, buying your case would be your best bet and like @Berg02 said it would be best to buy a new case so you can access the io and you can get a case with a front 3.5mm jack for your beats and just make sure you have the case's cables plugged into your motherboard.
If I were you, I would buy a gaming headset like http://gaming.logitech.com/en-us/product/g430-7-1-surround-sound-gaming-headset
Never had them, but there's so many option you could go with. Just to save my ears and everyone else's, don't use your beat's mic to talk to us.