Can you identify, by zip code, houses in Connecticut and Massachusetts
built before 1951 with a present value of  $200,000 +?  Can you
include, in your answer, domgraphic data such as income, race, etc for
given areas or geography?

There is a way to discover this but it would take weeks of work with
this method.  I have recently been searching for a home to purchase
and have been cross-referencing MLS (multiple listing service)
listings with the county tax appraisal records.  You could search the
tax appraisal records in each county of both states (if their records
are available online) to ascertain this information.

However, there are numerous appraisal values such as tax appraisal,
insurance (replacement cost) appraisal and realtor appraisal.  All of
these have been varied in my personal searches.  You might want to be
specific on which one of those you need.

PS  Also, tax appraisals include land value so a shack on 10 acres
might be in the price value you seek as well as a larger, solid home
on a tiny lot.  The tax appraisals usually break this apart somewhere
on the form.

Current market value is perhaps the best measure.  I'm not sure where
that falls among the three values you've mentioned.

I've been to the Census site and pulled infromation from this location
http://factfinder.census.gov/servlet/DatasetMainPageServlet?_ds_name=DEC_2000_SF3_U&_program=DEC&_lang=en

The Census is chock full of information, but how to weed through it is
a daunting task.  Is there something here that could make answering
the question easier?

My suggestion would be that you break down your question into many
smaller priced questions for each county in which you are interested. 
The task of doing this for two states is a daunting one.  Even
checking county tax records could take hours for each county because
the researcher would have to look at many records.  You might contact
the realty borad/commission in each state to ascertain if they have a
paid report of this nature.

Are you looking for this as a home buyer or do you have some
product/service you are targeting in these areas?

The Census Bureau does have figures from the 2000 census on age of
homes, value of homes and ZIP codes. It would be relatively easy,
using the FactFinder tool (factfinder.census.gov), to get an estimate
of how many pre-1951 homes, and how many $200,000+ homes, are in each
ZIP code in Southern New England. I doubt that the bureau has
cross-referenced home age by home value on the Web site. You probably
would have to contact the Census Bureau directly for that information.
The bureau has a list of staff contacts available at its Web site
(www.census.gov).

Also note that the Census numbers in those categories are based on
sampled data. That means the Census asked only 1 of 7 or so
householders about their homes' age and value, then extrapolated from
that sample to estimate how many pre-1951 or $200,000+ homes were in a
certain ZIP code. That means data could be somewhat skewed in smaller
communities.