Byrnie
Headphoneus Supremus
Regardless if the stereo sound is being converted into dolby digital surround sound 5.1/7.1, your headphones only have two channels to output. Any surround sound effects will simply a software simulation. DSP processing takes surround information and creates a new audio signal attempting to reproduce positional cues with two drivers. This process can degrade sound quality. Seems more like a gimmick. A pair of headphones with decent sound stage will be sufficient for positional cues in competitive FPS gaming needs.
If you are using your headphones for primarily for gaming & are more interested in "surround sound experience," just get something like the Astro Mixamp or Turtlebeach Earforce DSS or Creative Recon3D or X-Fi. End of story. No need for anything else.
A DAC ontop of a DSS sound processor is unnecessary as the DSS component already processes the sound. Every sound processing component inherently introduces a bit of their own coloration, so increasing the amount of sound processing components in your chain will not result in better sound quality. No need to double DAC or DAC+DSS sound processor.
You will not need an amplifier unless your headphones have an impedance greater than 100 ohms. An amplifier will have no impact on the sound quality as its function is to simply amplify the existing signal.
Basically, you shouldn't buy anything else no matter what anyone here tells you if your goal is surround sound. You can get by with either stereo sound on headphones with a good sound stage or good sound stage headphones + DSS sound processor. Amplifier if your headphone's have a high impedance.
I can attest to this as well given my own experience in the competitive FPSs. I will say that regardless of your headphone impedance an amp will help bring out more ambient sounds in your game (that's if the game has good sound). An amp isn't necessary though and a good set of cans will make the most difference in detail retrieval.