The lore for sizing shared_buffers generally specifies a suggestion in terms of a percentage of system RAM. Some advocate only 10-15%. The thorough academic exploration in the paper Tuning Database Configuration Parameters with iTuned at http://www.cs.duke.edu/~shivnath/papers/ituned.pdf found 40% optimal on a 1 GB system being tested on a wide range of queries. And occasionally, reports appear where 60% or more of total RAM turns out to be optimal.
Generally, if you have a server where OS overhead is small relative to total memory (any modern system with 1 GB or more of RAM), giving the database 25% of total RAM is a reasonable starting setting for shared_buffers in the middle of the effective range. It may not be ...
