Titles should ideally be questions.
But, more important is ensuring a question title contains adequate keywords relevant to the question to make the question a more likely search result in Google.
In your first case above, for instance, I might re-title it "Online data source or web site for forward foreign exchange rate quotes?"
A good title with the right keywords can often mean the difference between only dozens of search visitors, or tens of thousands. Case in point, this question got 55,000+ pageviews:
Ontario harmonized sales tax (HST) transition benefit rebate cheques / OSTTB: Who, when, how much?
The title above recognizes that individuals search on a variety of things:
- "hst rebate cheque"
- "osttb cheque"
- "harmonized sales tax cheque"
- "ontario hst cheque"
- "hst cheque how much"
So prefer verbose titles rich in keywords to short, terse titles. The question words are sometimes good search terms so they can help too. You'd be surprised (or not) how many people just type an English question into the Google search box!
(Addendum: Of course, sometimes a title can be too long. Be reasonable :-)