Postgresql Tuning

Hi,

I've got a PostgreSql database that serves a java aplication for about 15 concurrent
users that conect over modem. Mi PSQL version is 6.3.2. and I have it installed with default parameters.

Is there anithing I could/should parametrize to enhace performance?

Also:

Some of the conenctions to the database, specially when there's a high load of
them, stay opened as postgres processes indefinitively, as seem to block others,
even thoug every operation is doen inside a createConecction - conecction.close
block.

Is there any way to block by record instead of by table?, this would avoid my users
from being blocked while some other user is performing an intensive operation.

Thanks in advance.

Juan Alvarez Ferrando