Memory can be quite tricky to configure for 32-bit platform. Nonetheless for any platform MAX memory should be set. For dedicated server typically 80-90% of physical memory.
Server Memory Server Configuration Options
Optimizing Server Performance Using Memory Configuration Options
How to adjust memory usage by using configuration options in SQL Server
Suggested Max Memory Settings for SQL Server 2005/2008
How to configure memory settings for the 64-bit SQL Servers?
Best Practices for Memory Management of Multiple Instances of SQL Server 2008 R2
Single Instance vs Multiple Instances
Great SQL Server Debates: Lock Pages in Memory