SQL high and Low Memory usage

Hello Guys,

I have one of my servers with high memory usage, I read this article about high memory yet I noticed I have other SQL servers with very low usage.

the server I have with high usage, is always high, even if users not logging to the database or doing anything.

yet other servers in another location have 15 to 20% memory usage for SQL with all the users active.

Note: Both servers are Microsoft Windows 2008R2 with SQL 2005 (Legacy App)

I changed through the memory allocation option in the servers, all are using the maximum memory.

is there's an explanation for that?

I need to be sure if my first server hardware is enough if I have 70 users working on the same database.

Thank you in advance.