I'm looking for a comprehensive survey of art history.  I've heard
Janson's History of Art is the best, but looking at Amazon.com, I
notice that there are so many revised volumes that I'm not sure which
one would be the best to purchase.  Many are split into separate
volumes, some seem to be the same book with different cultures, and
people seem to gravitate more towards the 6th edition, even tho' there
is a newer 7th edition.  Which version & edition of Janson's Art
History would give me--definitively--the most comprehensive overview
on the subject of art, primarily western art, since the Renaissance? 
I should note that I know almost NOTHING about this subject.

If you're invested in this particular title, you might find this
article from the New York Times, commenting on the new 7th edition
(March 2006), interesting and helpful. (I'm not sure why this is still
accessible; usually articles are archived after 7 days. Hope the link
works for you.)

http://www.nytimes.com/2006/03/07/arts/design/07jans.html?pagewanted=1&ex=1299387600en=b5347cba58738b42ei=5088partner=rssnytemc=rss

[Here's another link to the article on an art gallery site, just in case:
http://www.sgallery.net/news/03_2006/08.php.]

Janson's is the book taught in most schools and one of my friends just
finished the Modern portion of the new 7th edition.  It is the most
comprehensive overview .I am an art dealer and think that if you just
want to know about the History of art, you might want to check out EH
Gombrich, the Story of Art. It is a little easier to get through. I
hope you will get a chance to go to museums and see the works that you
read about..Good luck