How to improve sql server 2005 by 300 people visit [on hold]

I have took a server now，I can't modify the source code.the service is to slowly.

IBM server windows 2008 sql server 2005 mem 16G CPU 8*1.2 300 people visit

There are more than fifty table database，The database each table has more than one hundred fields

How do I do to improve the performance of service？

Use two servers or other ways thanks