IMDb rankings can be kept up-to-date by selecting all the movies with a rank, and updating them. Even if not done very often, there will only be a small number missing from the end of the sequence. If it matters, these can be identified and added (e.g., when I did this yesterday, I had to manually add numbers 203, 230, 236, 245 and 250).
If starting with a database with no IMDb rank recorded, this method obviously won't work. The list can be copied into Excel, parsed, and imported to PVD. Unfortunately, the import plugin will not match records using the URL (which can be extracted from the hyperlinks in the list), so it's necessary to make sure the titles in the import are exactly the same as those used in PVD. This is almost as tedious as entering all the ranks manually. So in this situation, I would suggest selecting all the movies with a Rating greater than 7.9 and updating them. That may be considerably more than 250 records, but it will get all the Top 250 (that are in the database).
To "dynamically track" the rankings, a separate plugin would be required (or maybe a script, but I don't know if a script is able to interact with the database in the manner required). This would get the list and update the database with the current rank and votes. It might also add any movies that appear on the list but are not in the database. This would be much faster than updating 250 records, and could be run daily or as required. Perhaps it would have an option to run the update every n days.
