hi Brian
In a broad match search using google adwords tool every keyword is searched for so an adwords search for Evening Dress would look for the words Evening + Dress either singular or in any combination,
You have to remember that these searches are aimed at google adwords advertisers NOT for SEOIn my opinion I wouldn't worry too much about search volume because over time, if you do quality content, then you will attract visitors for keywords you haven't even optimised for.
Exact and phrase are more to do with advertising with google adwords than optimising pages and allow advertisers to concentrate and streamline their campaign.
I don't know of anyone who searches using exact [keyword] because it doesn't make sense and it doesn't make any difference
Between the two I think Phrase "keyword" is possibly the best one to search with as this is used by a few people.
I've found that the vast majority of people just type in the keyword or keyword phrase
Google search results Evening Dress - 35,900,000
Google search results [Evening Dress] - 35,900,000
Google search results "Evening Dress" - 22,900,000
If you want googles definition and explanation then here's the link
http://adwords.google.com/support/aw/bin/answer.py?hl=en&answer=6100Hoping this helps
Eliot