Well, Sennheiser's HD600 series used to be their top-of-the-line audiophile-grade headphones (£260 a few years ago), so they're going to be some of their better ones (although I seem to remember them being 300 Ohm resistance, so make sure your sound device can handle that). Also, the HD600's are open-backed, so whilst they'll have a good sound stage, they'll leak sound out, and let sound in (i.e. have little noise isolation) - not saying this is bad, just something to bear in mind.
From talking with a co-worker who's an audiophile, they reckon Sennheiser's headphones (unlike their in-ear monmitors) are a bit lacking (flat), especailly when compared to others (they rate Grado), and not as comfortable as they look.
From what i've read around, gaming headsets are generally of compromised quality, and you're better off getting "real" headphones.