Surround sound and Headphones, IMO is not comparable. They both offer something different which is useable in completely different situations. You'd want a 5.1 setup perhaps if you were watching movies, however in relation to klipsch's Promedia crap, you are missing absolutely nothing. In comparison, your headphones are so much better at reproducing sound it's rediculous, so it's pointless even trying to get into that.
To elaborate on the "5.1" thing, this simply indicates a five speaker single sub setup. You would have three speakers in the front, left right and center, and two in the rear, left and right and a single subwoofer reproducing sounds. To get the full affect, you need DD encoded material (mainly movies) -- otherwise it's just stereophonic coming through a boatload of speakers, or some "simulated" affect that the decoder has built in if just playing standard music CDs which usually sounds horrid.