I'm looking for information on age-gap relationships and marriages,
specifically where the woman is older than the man. I've done some
research myself and come up relatively dry, so I know not to expect
the world. I'm asking for a number of things below, in the hopes of
getting  relative thoroughness.

-I'm seeking research with quantifiable results about the % of
marriages by years (or tiers) of age difference in partners? Is this
broken down by which gender is older (specifically older woman)?
Cultural/Geographic breakdown?
-Information on divorce rates as linked to years of age difference in
partners, again ideally broken down by which gender is older. If
available, data on whether the age gap was the cause.
-Other 'measures of success or failure' as based on age difference.
a nice to find is, but less important:
-Survey Data on perceived appropriateness of age differences, by # of
years or tiers - specifically where the woman is older.

Hi gindo:

I wasn't able to find all the information you wanted, but here's what
I did find:

Article: Australian Social Trends 1997
Overview: This paper has some interesting data about age differences
between brides and grooms, including a nice graph that shows data for
1995 vs. 1974:
Quote: "In 1995, about two thirds of first marriages involved a
bridegroom marrying a younger bride, and in 14% they were the same
age. For the remaining 20% of first marriages, the bride was older. By
comparison, in 1974 only 11% of first marriages involved a bridegroom
marrying an older bride.
However, the age gap between bridegroom and bride was generally
smaller when the bridegroom was younger than the bride. In 1995, 15%
of first marriages where the bridegroom was younger than the bride
involved an age difference greater than four years. In comparison, 31%
of first marriages in which the bridegroom was older than the bride
involved an age difference greater than four years."
URL: http://www.abs.gov.au/ausstats/abs@.nsf/94713ad445ff1425ca25682000192af2/545b78dcdf028c71ca2569bb00164f62!OpenDocument

Article: Age Difference Between Brides and Grooms in Australia
Overview: This short paper examines the trends in age differences
between grooms and brides from 1921 - 1995. There is an excellent bar
graph (Figure 2) that shows the trend towards more "older woman"
marriages.
Quote: "[In 1921] 16 per cent of marriages involved a bride two or
more years older than the groom, compared with 21% per cent of such
marriages in 1995."
URL: http://www.aifs.org.au/institute/pubs/fm/fm49lx.pdf

I hope another researcher can find everything you wanted.

websearcher-ga

Gindo --

Such marriages are referred to as May-December marriages in the U.S.
and a search in Google using that term only turns up 61 references. 
Many of the references are advice columns and letters expressing
opinions.

Though I don't have time to research it more completely at the moment,
an alternate Google search strategy might be the following, which
turns up thousands of documents, many related to analysis of changes
in marriage age:
variance + age + marriage

The U.S. Census Bureau seems to have limited data, but its "Families
and Living Arrangements" report, which is constantly being updated,
has data on spouses who are separated by 6+ years in age.  In the 1999
report, couples in which the man was 6+ years older accounted for 20%
of the married couples; in 3.3% of the cases the woman was 6+ years
older:
http://www.census.gov/population/www/socdemo/hh-fam.html

Good luck!