Should I get a sound card or not?

I am getting a pair of Sennheiser 555 headset because of all the tons of great reviews of them and they look extremely comfortable which is the selling point for me.  I have Windows 7 64 bit system and audio can go up to like Studio Quality sounds if I wanted to.  I honestly can't tell a difference whether I have it on CD quality or Studio quality but that is just me.  I want to be able to detect footsteps and gun shots in FPS mostly and other sounds in Company of Heroes like voices and gunshots.  The only sound card I had was years ago to accompany my 5.1 surround system, but now I have a 2 channel speaker system and a 7.1 audio ports on my motherboard which pretty much come standard on today's motherboards for the most part.  Now people have been telling me they get much better sound with an X-Fi sound card or something similar.  I saw Fatal1ty had a Xfi sound card on newegg for like 140 with free shipping and it looked pretty good, but considering I already have a 7.1 built into the motherboard, is it worth upgrading?  I don't know that much about sound cards and sound in general.  The best set of headphones I ever got were the Razer Barracuda's and I used them on a stock sound card and I picked up so many things I couldn't hear from the 2.1 speaker system that I could with those.  They were extremely comfortable as well, but I just wanted to improve the sound quality so I got the Sennheiser 555 because the reviews were amazing and they look extremely comfortable.  Any help would be very much appreciative.
Movie Stars

Solution: Should I get a sound card or not?

I have a Creative Labs SoundBlaster X-Fi XtremeMusic and onboard 7.1 sound.  I no longer use the Sound Blaster (other than sound processing features like adding echos, it does nothing I need.)  I often had driver problems with it.  Additionally, it couldn't output a game in 5.1 or DTS.  The onboard has no problem doing that.

If your onboard sound card works, I wouldn't bother adding a new one.