tuning my db

I am new to Postgresql.

I want to tune my Postgresql config, I used pgtune (http://pgtune.leopard.in.ua/) to set my parameters, is it the best way to set them?

I want to improve the database performance (time)  choosing the best configuration for my db.

The parameters of my system are: Ubuntu LTS 12.04 and 3635 MB of RAM.

And then : why does "ORDER BY" increase the query execution time ?

I have 3 dbs, the first one is 108 MB, the second  one is 1200 MB and the last one is 11 GB, why in the first two dbs to solve a query does it use the bitmap heap scan, instead in the last one it uses the seq scan?

Thank you

Valentina