I know its 5.1, but if you can find the logitech z5300e, i suggest picking them up. They are very very good mid range speakers and the sub packs a real punch. My only complaint is that the sub is attached to the receiver and the receiver doesn't have spdif/coaxial input.
i would suggest staying away from 7.1. although it is a good idea, actually setting it up can be quite difficult. plus, very few sources actually support 7.1, short of some games. no music is encoded in anything over stereo, and i don't even think movies support 7.1.
7.1 channel capability is there, but I really don't much media (games, movies, music, etc) is even done in 7.1 yet. 5.1 is really only good for movies, and certain games anyhow. Most games you're still looking at fake surround sound (duplicated stereo).
I would agree with other responders that 7.1 vs. 5.1 is not much difference.
As to the best unit, look at the size of the subwoofer speaker. The larger the better. A larger speaker will be able to reproduce lower frequencies. I would not really consider the wattage as a factor in the $100 range.