I thought I would also point out that you can get very convincing "surround" sound out of a regular pair of stereo headphones, probably equal or better to the surround effect of a pair of "true" 5.1 headphones. The reason is because in order to cram 4 speakers into each side of the headphone, the speakers need to be smaller which sacrifices sound quality.
If you have an x-fi sound card, I would recommend that you try their CMSS 3d sound (i think thats what its called) with a pair of headphones you already have and see if you like the effect, if it sounds surround to you, then you would probably be better served buying a quality pair of stereo headphones and using that.
I was looking for the same thing you were in november, and I finally settled on a pair of sennheiser headphones (which im sure sound better than most if not all 5.1 headphones) and an Asus Xonar sound card, which includes the option to use Dolby Headphone technology (which basically takes a 5.1 source, and encodes each channel then downmixes it). I will tell you that I came out of it with an awesome sounding pair of stereo headphones, and for games, I have full surround sound, I can hear things coming from all directions, including behind me.
Just some food for thought. You will still probably be well served by buying a 5.1 pair, just after all my research I went a different route.
