I agree with the above comment about this being a subjective question. With that being said I will provide you my personal opinion after having used 5.1 digital surround sound headphones for gaming for around 2 years.
I can't recommend them enough if the money isn't an issue. The headphones help substantially in a couple different ways.
First: The sound clarity is amazing. You'll end up hearing things you normally wouldn't have heard. Footsteps seems amplified and its easier to tell the direction of gunfire.
Second: Direction. In addition to the location of gunfire, the 5.1 surround sound makes it easier to identify the direction of the footsteps. This helps prevent people from sneaking up on you.
Third: Late night gaming. It's useful to play a FPS at ear splitting volume but never wake anyone up. On late nights your significant other will thank you.
Finally: The most important aspect of gaming headphones. It blocks out ambient noise. This is true with any headphones and will greatly improve your game. Whether you realize it or not your body focuses on ambient noises and by blocking them out I have found my concentration for the game is greatly improved.
As with any other recommendation, take this with a grain of salt. But I have personally found gaming headphones to be invaluable in my FPS game.