There are 7.1 headsets, but in reality, they wont be good. Problem is the way the brain decodes sound.
Even if you do have the amount of speakers inside a headphone to be a real 7.1 headphone, the distance to your ear is so small that the actual effect will be minimal.
Assuming a 7.1 speaker system is out of the question, a good stereo normally beats a low quality 5.1/7.1 headphone, even in positioning.
I own a 5.1 speaker system that costed me 279 Euros (Logitech Z-5500) + Creative Titanium HD as sound card (69 Euros) and a stereo headphone system that costed me over 1000 dollars, and when i play games taht have exquisit 5.1 quality i still enjoy the speakers more.
However when audio quality is superb, i get the headphones everytime.
Games that have great 5.1 from my point of view: Unreal tournament sereies (even the first one was great at this), mass effect 2, Just Cause 2... Amnesia - The dark Decent if you have the balls, Stalker series.... Cant think of any others right now.