There is little difference between 5.1 and 7.1 mostly if its for gaming. I recommend a good sound card like the xonar dx and a entry level audiophile grade set of headphones and a Zalman clip on mic if you want to get the most for you money.
If you want you can get true 5.1 surround sound by getting the Roccat Kaves which are right in your price rang but
If your not that picky about the quality of your sound then you can get a usb headset but there not bad its just that you could have more with a real decent setup for games, music, movies or anything else. For a usb headset I heard that the Zowie Hammer headset has really good headphone drivers.
Keep in mind if you want to get most out of your sound with a soundcard and audiophile grade headphones you will need to get a dac.
Good luck on your decision