There's been some recent, interesting discussion on the pgsql-performance mailing list on using PostgreSQL as an in-memory only database. In other words, you basically want to use it as a cache, similar to the way that you would use memcached or a NoSQL solution, but with a lot more features.
If you're interested in doing this, you'll need to configure the system so that you have a convenient, automatic way erase the database cluster and reinitialize it (using initdb) after an operating system crash. Per discussion on the mailing list, for best performance, it seems best to set up the data directory on a tmpfs and configure the following parameters in postgresql.conf:
With fsync=off, and most likely also with full_page_writes=off, your database will not be crash-safe - but you don't care, because you're planning to start from scratch after a crash anyway. If you're familiar with postgresql.conf parameters, setting synchronous_commit=off might seem redundant if you've already set fsync=off, but testing reveals that it still boosts performance. Turning off full_page_writes and bgwriter_lru_maxpages eliminates I/O that isn't needed for this use case.
On a related note, Gavin Roy gave a talk at PGCon comparing the performance of PostgreSQL with fsync=off with a number of NoSQL databases. The results were pretty good, but there might even be room for improvement with some additional tuning.
If you end up testing a configuration along these lines, please post a comment here or on the pgsql-performance mailing list with your experiences.