Is it research or plagiarism?
Nancy Sanders in her book,
Yes! You Can Build a Successful Writing Career, says:
"The general rule of thumb is to use three resources for each fact you state in your manuscript.
A rule to remember is: If you just use one source, it's called plagiarism. If you use three sources, it's called research."
Do check out her blog, called Blogzone. There's a link to it on her website! It's full of good advice -- currently she's doing a series about internet marketing.
Another nonfiction writer reminded me that even three sources can be reporting the same erroneous fact. She advises people to research deeper, as close to primary sources as possible -- and avoid using quotes from sources of questionable reliability, like Wikipedia.