Probably won't find any 5.1 sets for under 100, but the Turtle Beach DPX21 and Tritton AX Pros are around $150. You can find them cheaper, gotta look around, but I personally have the AX Pros and they are phenomenal. They have 4 speakers in each ear, so they have true surround sound. The Turtle Beach ones have one speaker in each ear, but the DSS unit it comes with simulates 5.1. The DSS unit can be used with any stereo headset to give you the simulated surround sound. They give you a bit more versatility, but they don't have the same depth of sound that the AX Pros do.
If you can handle just stereo for now, the Turtle Beach PX3s or Logitech F540s are great wireless headsets. They both offer multi input support so you can have both the Xbox and PS3 hooked up at once without having to switch up cables every time.
I have the AX Pros and Logitech sets and recommend both. The Logitechs are probably the cheapest on Amazon, so if you want wireless and ease of access for both systems, Logitech all the way. You can also pick up the DSS unit later to enhance the wireless sets.
Good luck man.
