You shouldn't run a very busy forum on shared hosting as well. When I say very busy I'm meaning 750+ post's a day.
we cannot tell busy forum with +750 posts a day, it's nothing. The busy forum is +5,000 posts a day
Last few days I reached to +3,000 per/day I have about +7,000,000 posts until now and its growing very fast , I have to say that I have to tune up my SQL for time to time ...
Thanks