5.1 is overrated. It is NOWHERE near speaker 5.1 surround. I've tried the Zalman 5.1's and the Medusa 5.1's and neither of them cut the mustard for me. Plus the Medusa 5.1's are known for having a tinny sound from the rear speakers (i.e. no bass) which is ok for movies, but when your playing a game and you like to hear bullets go off behind you, its kind of crap when they sound like someone throwing stones at a tin roof.
Go for the Senns... i'd personally recommend getting a really good soundcard, even the most mediocre headphones will sound amazing with the right soundcard. Personally i'll be getting a pair of HD55's
The soundcard is onboard...