effective_cache_size X shared_buffer

I'm thinking about increasing the query cache for my PG 9.2 server.

I've got a project happening, which is doing lots and lots of writes and reads during the night, and in the morning I see PG cache warming up again, as all the cache "was used" by those write and read tasks.

So my environment gets very slow for a few hours, until the queries used on a daily basis go to the cache.

Question:

Should I increase effective_cache_size or shared_buffer? What's the difference between them?

Thanks

Patrick