In general, your object-oriented code will be slower than non-OO code. But that's not the whole story, because the slowdown is not very significant, and is far outweighed by things like disk access, database access, etc. So on the whole, it would make things easier for you, probably, to "go OO", at least conceptually, then I'd say go for it.

You also asked:

    > Assuming I can't get SQL, would I see a performance
    > increase if (e.g.) I moved to DBM from the
    > tab-delimited flat files?
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Yes, so long as you use them correctly. By that I mean, you need to structure your data in such a way--namely, key-value pairs--that the benefits of using a DBM will be displayed. For doing lookups on data, getting that data out of a DBM, where you have some unique key into the DBM, is much faster than searching through each line in a text file, trying to find the data. But you have to get your data set up in such a way that this is the case.

And yes, I would recommend using Berkeley DB. Get version 2, because I've heard about memory leaks in 1.