I'm not sure that I can completely follow your question, but I'll try to give a good answer here:
FamilyTreeDNA (and Y-Search) allow you to compare your results to other people that have taken the test, whether those people are in your surname or not. You won't know for sure how many matches you have received until after you've taken the test.
You could use Y-DNA to compare ancestry to your GGGG-Grandmother's husband (by testing a direct male decendant). You can also use mtDNA to test for your GGGG-Grandmother's line (by testing a child of a direct female decendant).
A lot of questions are answered in the Frequently Asked Questions part of our website:http://www.worldfamilies.net/faqs.html