I agree with @Bradley.
XML is very slow and not particularly useful as a storage format. Why bother? Will you be editing the data by hand using a text editor? If so, XML still isn't a very convenient format compared to something like YAML. With something like SQlite, queries are easier to write, and there's a well defined API for getting your data in and out.
XML is fine if you need to send data around between programs. But in the name of efficiency, you should probably produce the XML at sending time, and parse it into "real data" at receive time.
All the above means that your question about "when the overhead of a database is justified" is kind of moot. XML has a way higher overhead, all the time, than SQlite does. (Full-on databases like MSSQL are heavier, especially in administrative overhead, but that's a totally different question.)
