During this Pentecost week, I just happened to find a post by Dr. Taylor Marshall on Canterbury Tales, which deals with why the proper English translation of Spiritus Sancti is "Holy Ghost." It is a very interesting article, and I encourage you to read it thyself.
First of all, the Douay-Rheims Bible, the official English translation of the Catholic Bible, says "Holy Ghost" most of the time, but occasionally says "holy Spirit" (notice that "holy" is not capitialized). My hand missal says "Holy Ghost." My parish describes the third person of the Most Holy Trinity as "The Holy Ghost."
While the word "spirit" is a litteral translation of the Latin spiritus, it has been received in Sacred Tradition that the phrase Spiritus Sancti translate as "Holy Ghost." Dr. Marshall also mentions in the article most English-speaking saints have used the term "Holy Ghost" in their sermons. The Baltimore Catechism also uses this term.
Please note that the common use of "Holy Spirit" among Catholics is a Novus Ordo and liberal Protestant invention; Traditionalist Catholics and Anglicans have retained "Holy Ghost" when they speak English, liturgically or conversationally. I'm not exactly sure which term Eastern Orthodox Christians use when speaking English, however.
This comes to one point: my mother continues to use the term "Holy Spirit," even after several times trying to tell her that it's not proper. When I get married (if that ever happens), my wife and children will be using the term "Holy Ghost," regardless of what other people say.