If I want to buy a copy of the Bible, which version does one buy that would be the official Roman Catholic version?
It seems the Kings James version is the most popular, is that the one?
Will this contain both the New and Old Testament?
The Official Bible is the Vulgate, translated into Latin from (mainly) Greek by St. Jerome in the 4th Century after the canon was finally established.
The Council of Trent's Fourth Session defined the old Vulgate as authentic (source):
Moreover, the same sacred and holy Synod,—considering that no small utility may accrue to the Church of God, if it be made known which out of all the Latin editions, now in circulation, of the sacred books, is to be held as authentic,—ordains and declares, that the said old and vulgate edition, which, by the lengthened usage of so many ages, has been approved of in the Church, be, in public lectures, disputations, sermons, and expositions, held as authentic; and that no one is to dare, or presume to reject it under any pretext whatever.
If you're looking for an English translation, any Bible with an imprimatur should be good. I wrote another answer here a while ago about whether Catholics can read the NIV that might be of use to you. The point is, Catholics should look to the Church for official interpretation no matter what the translation is.
There is a list of several approved English translations published by the USCCB. The Vatican has a copy of the New American Bible on it's website. These translations include the New Testament and the Roman Catholic Old Testament Canon.
The King James Version is a popular translation for English-speaking Protestant Christians. It contains the New Testament and the Protestant Old Testament Canon. There is a subset of conservative English-speaking Christians that consider it to be the only authorized version of the Bible but for most English-speaking Christians it is only one of many popular translations.
THE official Bible in the Roman Catholic Church is the Latin Vulgate Bible or it could be the Enlish version which contained 73 books.46 books in the Old Testament and 27 in the New testament. Those Bibles comprised only 66 books both Old and New testament are not Catholic Bible. For the Catholic believers much better to use the Catholic Bible than to use the protestant Bible which have been a lot twisted verse from the original Bible called Septuagint.