all depends on what you define as 'best'
what i mean is that if you're a coding wizard or something like that you'll probably go linux or even unix
if you look at user friendly interfances etc ... i guess i'd say OSX Panther (for now)
all would depend on if your OS supports the Headset or not ...
my guess is that OS devs won't support something that isn't made for it ... that is if you are talking about a mobile phone headset. But i could be totaly wrong
Technicaly it IS possible
i'm using a dual monitor configuration on my G4 powerbook, using my old 15" CRT at 1240ish in res. I don't know of it's an option or setting on windows based os'es but i can switch back to single display to play games, so that only one monitor is being used, and so, can use all the GPU power