shared_buffers and effective_cache_size

Hi,
I just got a new Penguin machine with 2 processors and 2G RAM. After
installed postgres 7.2, I started to modify postgresql.conf to get max
performance. I read Adm Guide and Bruce's article: PostgreSQL Hardware
Performance Tuning, in my understanding, shared_buffers is similar to
Oracle's db_block_buffers, but also sometimes I feel it sounds like oracle's
SGA. When I reached effective_cache_size, don't know what's that, docs can't
help, it seems for optimizer, but how big should I set? no feeling! Does
anyone can compare this paremeter to oracle's so I can understand better.

If I set SHARED_BUFFERS = 131072, what effective_cache_size should be?

Thanks!

Anna Zhang