welcome to the site!
please use the search button before starting a new thread. a lot of common questions can be answered by doing a little research first.
here is a link to one you might find interesting. http://www.yourcobalt.com/forums/engine-mods/11224-jet-trifecta.html
IMO the jet chip is not worth the money. they do work, but not nearly to the point a true tune will. it does have its positives though. they can be removed for warranty work being one of the best.
not really sure about hp gains. as with all mods, some cars will react different than others for various reasons. level of existing mods etc...