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I know you can adjust the order of the categories, but I think the topics are in the order they were created. One way to alphabetize them may be to recreate them in the order you want, then delete the old ones. Of course, you lose posts this way. You might be able to transfer the old posts to the new forums with an SQL UPDATE command of some sort.
It would be very nice if we could assign a weight to the forum topics, not just the categories.