I am guessing he is after a sound card not a dedicated DAC, and with that headset you are unlikely to notice any upgrade in sound quality, I have found most headsets to be mediocre at best and some are flat out ridiculously over priced.  Consider the headset is worth £65 approx in UK, when you take into account the Razer brand and the mic these are equivalent of £30 headphones at best, and we are talking about a DAC having a negative impact on positional queues.  Reality is the money is being wasted, on-board sound can do the job plenty good enough for a gaming headset.

Buy better headphones before wasting money on anything else.

Thank you all.
Again - I'm really a beginner in the world of hi fi and sound

A good DAC+Amp can make a huge difference, if your headphones are up to the task. With my HiFiMan HE-400 headphones (admittedly harder to drive than a gaming headset), using a decent headphone amplifier provides a huge improvement in quality over just driving them from my MacBook Pro. Adding a USB DAC instead of using the internal sound card was an obvious improvement, but not as dramatic as adding an amp.
 
That said, gaming headsets aren't generally built with sound quality in mind, so your best bet would probably be to spend that $100 on a nice set of headphones.

Rather than spend $100 on a headset and $100 on a dac amp you still get more bang for buck with $200 headphones and on-board sound.  And talking about HE-400 for gaming is using a porsche for the school-run capable but overkill

tablix said:

  Rather than spend $100 on a headset and $100 on a dac amp you still get more bang for buck with $200 headphones and on-board sound.  And talking about HE-400 for gaming is using a porsche for the school-run capable but overkill

Click to expand...

I'm not suggesting he go out and buy a set of HE-400s. My point was, yes DACs and amps make a difference, but not without good headphones.

ab0tj said:

 
I'm not suggesting he go out and buy a set of HE-400s. My point was, yes DACs and amps make a difference, but not without good headphones.

Click to expand...

 
... but the difference isn't that much what two different pair of headphones brings ....

jiiteepee said:

   
... but the difference isn't that much what two different pair of headphones brings ....

Click to expand...

... which is exactly why he'd be better off spending the $100 on a different set of headphones.