Only index open threads for search?

Hey guys, with 3MM posts, my site is doing horible with search. I'm trying to figure out a few creative solutions to ease my server pains...

Is it posible to index only open threads? I'm thinking of installing a hack that will auto-lock threads that go for x amount of months with no activity. If I can keep my search index smaller, I can get better performance out of my server.

Any way to do this?