Real 5.1 headsets are usually not THAT good, or you have to spend lots of cash.
I would go with a good set of cans (not headset) from sennheiser or beyerdynamics, and get a seperate mic, and a dedicated soundcard.
The HD558 are pretty good and can be easily modded (removing the rubber strip inside the housing) to get the sound of the next bigger ones (HD598).
Get something like the asus Dx or Dsx card, and use the Uni drivers (google).
Im using the 558 with a Dx, and frontstage as well as sound positioning is much better than with any headset i ever had, incl a real 5.1..