a cardioid pickup pattern microphone would be ideal as it would help eliminate pickup from the rear. the blue snowball and atr2500 both have this.
be careful of feedback loop! if you have your system set to play microphone audio live as you record on your speakers you can quite quickly get a loud surprise. if you do not have your pc set to play recorded audio from the microphone live, this will not be much of a problem.
it would be better if your speakers were to the sides and behind, not directly behind the mic if possible.
the snowball is a good mic. the atr2500usb is another.
as long as you keep volume fairly quiet or low levels while using the mic you shouldnt have big issues. if you have your volume at medium to high levels you might start having some noticible issues though.