cheaper sets:
5.1 - X-530
7.1 - if you are looking for cheap just go with 5.1
but if you desire cheap 7.1 then get the Creative Inspire P7800 I don't recommend it tho...
mid-range price:
5.1 - Logitech Z-5300e (not bad a price for the power)
7.1 - not really a mid-range for 7.1 that I can think of
high end
5.1 - Logitech Z5500 or Klipsch Promedia Ultra
7.1 - Creative Gigaworks S750