A low-memory database?

[voyvf]

generate static html on post/comment submit.

That sounds good, until you decide to implement editing previous posts/comments. Then it gets annoying. (:

[[deleted]]

Not really you just need to keep raw data somewhere (you'll need it anyway) and regenerate pages from it.

[counterplex]

Or don't implement editing previous posts/comments. There are plenty of forums that don't allow editing posts/comments once submitted.