param max_connections

Hallo,
I am using PostgreSQL 7.4 on UltraSparc machine and I have a touble with
max_connections param.
I must set only max_connections to 8, when I set more, I got error for shared
memory.

Most connection is from J2EE application server and here is no problem, I am
using connection pooling and I have defined maximum connections for each
datasource here, but there is an old application written in php too, and here is
a problem, sometime connections runs over this param value.

Is this param familiar or I have a problem somewhere in configuration / compilation?

Server has 1 CPU UltraSparc II and 512MB RAM

Thanks for help,
Jiri