Ive owned the 550's and they definitely rocked, especially with music. MD1032, Cambridge Soundworks and Creative is the same company and they offer one of the best bang for your buck on HTIB's and speakers plus they have a great 45 day policy to let you try the speakers in home and if they are not to your satisfaction, simply return them (hifi.com). I prefer Cambridge over Klipsch because they are much more musical and natural, youd be suprised what that little neodymium satellite can do. As for the Z680s, google on Cambridge and Klipsch history. I just have a hard time letting a company that manufactures primarily pc accessories to also replace my speakers, even if it is just for pc use.
Heres one of the only head to head articles Ive read between Cambridge and Klipsch
http://www.extremetech.com/article2/0,3973,73282,00.asp
neosho, only your own ears can judge which set is better, all these reviews are just a matter of opinion.. and I also throw in mine
Klipsch Promedia - if you watch alot of movies
Cambridge 550s - if you listen to alot of music