First of all, you're talking about 600 million pieces of information, which amounts to 'large amounts of data' and databases are intented to maintain 'large amounts of data', so I'd recommend a database backend.

Second of all, how do you generate the id's? If it can be sequential, 6 digits (0-9+A-Z) would give you over three times the number of id's you'd need.

So I guess it depends on what other criteria the id's need to adhere to. Let us know; we're dying to find out... :)