Make a note...

Does the copyright information appear on the Barbie's tush and NOT on her lower back? If the answer is yes, then your doll would be considered vintage (1959-1972), and these points will help you identify which doll you have. (we will assume for the time being that your doll's head was not switched and is the one that actually was issued by Mattel on the body you have...we will also assume that your doll was not used as a lab experiment by a little brother, or as a voo-doo doll, etc...)
If her tush says "Barbie" in script and then some Roman numerals after that, it is definitely vintage and probably valuable to a collector somewhere. The "Barbie" can be followed by either an "R" or a "TM". The earliest Barbie dolls made had the "Barbie TM" mark on the right buttock, and this mark was used for the #1 ponytail, #2 ponytail, #3 ponytail, and #4 ponytail. The mark was changed to "Barbie R", and this mold was used for the later #4 ponytail, #5 ponytail and #1 bubble-cut dolls. If your doll bears this copyright information, she is probably worth holding.
If the copyright information says "Midge 1962/Barbie 1958", followed by the years, then it is the body Mattel used for dolls issued 1962 and after. Later bubble-cut Barbie dolls, #6 ponytail and #7 ponytail, 1969 and 1970 Standard Barbie dolls, plus Midge dolls were issued with this body. Again, these are all collectible and probably valuable if in good condition. For the bubbles, ponytails and Midge dolls, the vinyl was the regular tan color. For the Standard Barbie, it was changed to pink vinyl (same body mold, just different-colored vinyl).
If the copyright says "1958 Mattel Inc." or "1958 Mattel Inc. Made in Japan", then you have either a Miss Barbie (issued in 1964) or what collectors refer to as an American Girl Barbie, (issued in 1965 and 1966) which is one of the most popular and collectible of all the Barbie family dolls. These dolls can command prices sometimes as high as the very early ponytails if in good condition. They are most notable for the fact that their legs bend at the knees with an internal mechanism, and this sets them apart from all the earlier dolls.

If the copyright says "1966 Mattel Made in Japan", this is also a good sign. Barbie got a new face in 1967 and these dolls are referred to as "Twist-n-Turn" Barbies, or "TNTs". The vinyl of their heads and bodies is pink, and they have a unique waist cut at an angle so that when they twist in one direction, their shoulder comes down toward the opposite hip. (This is quite different from the later 1966 copyright dolls, whose waists turn, but NOT at an angle. They just swivel from side to side WITHOUT the angled cut that makes their shoulders dip from side to side.) Another doll with this same copyright is the Malibu Barbie, and she was made with the same body mold, but with dark tan vinyl. She will also be marked "Made in Japan", and this is very important. Later dolls also carried this 1966 copyright, but they were NOT made in Japan and their value (speaking generally) goes downhill from here. Of course, there are exceptions to this rule (as always), but most of the very valuable and collectible Barbie and family dolls were made in Japan and bear this mark.
If the copyright information says "1965 Mattel Made in Japan", then you probably have a Barbie family doll such as Francie, who is extremely collectible. She has slimmer body proportions than Barbie and is a bit shorter.

If the copyright information says "1963 Mattel" then you probably have a Skipper doll, Barbie's little sister, who is also quite collectible.

Does your doll have real rooted eyelashes? Besides the copyright information, this is another very good sign. The dolls of the late 60s and early 70s had real eyelashes, and almost any of these would be considered collectible (just remember that some dolls from this same period did NOT have rooted eyelashes, so it does not mean that no eyelashes means no value...).

About the "Made in Japan" stuff...It is generally true that if a Barbie doll (or family doll) was made in Japan, it is collectible and probably valuable to someone. But as stated above, there are always exceptions. Some of the Barbie family dolls such as the later Skippers, Francies, Staceys, Christies, Julias, etc. were made in places such as Taiwan, Mexico, Korea, Hong Kong, etc. For these dolls, though, you would generally know that they are collectible because they look markedly different than your run-of-the-mill current-day Barbie dolls, i.e. they have real rooted eyelashes, are black dolls, are talking dolls, are smaller than Barbie (like Francie & Skipper), etc.

The most prevalent mistake made by people in determining a Barbie's value is thinking that the "1966" copyright information means something. This is NOT TRUE. There are millions of dolls marked 1966 and they are basically worthless to collectors. Even dolls made in the 1990s have this 1966 mark, so don't be fooled. But each doll should be evaluated on a case-by-case basis because there were foreign-issue dolls and others that are exceptions to these rules.

Please keep in mind that all of the above is just a GENERAL OVERVIEW, and that I am just a collector, not an expert (I could collect for 100 years and still not know everything there is to know about these dolls). If you need help identifying a particular doll, the best thing to do is either find a collector, or purchase a reference book. There are many available on the market today, but my highest recommendation goes to Marcie Melillo for her book "The Ultimate Barbie Doll Book". It is my bible (for the most part) in identifying vintage dolls and is an invaluable resource.