Post table is too big, how to reduce or split it?

[Carnage]

My instinct on how to deal with this would be two or more tables

archive_0_post, archive_1_post ... archive_n_post and live_post

Modify your board to make new posts to live_post; periodically move posts from live_post to archive_n_post based on last use (eg if they are in a thread that is updated often leave them there even if they themselves are fairly old)

Create a view called post joining all the archive tables and the live table the view should be updatable so post edits etc will still work. I don't know how sphinx deals with indexes, you may have to add indexes for each table separatly or have an index on the view.

I don't know what the performance will be like as I don't know exactly how mysql deals with views - it may well get worse but in your situation, this would be the first solution i'd look at in detail as it requires the least code edits/loss of functionallity.