SQL Server - How to determine ideal memory for instance?
A good starting point is best outlined by Glenn in his article : Suggested Max Memory Settings for SQL Server 2005/2008

Best is to baseline your database server usage during your full business cycle as that will give you the best number based on your workload using below PERFMON counters :


SQL Server:Buffer Manager\Page Life Expectancy 
SQL Server:Buffer Manager\Page reads/sec 
Physical Disk\Disk Reads/sec
Memory\Available Mbytes
SQL Server: Memory Manager - Total Server Memory
SQL Server: Memory Manager - Target Server Memory


Note: If you are going to use any formula or online calculator to calculate SQL Server memory configuration then best is to read Beaware of Wow… An online calculator to misconfigure your SQL Server memory ! - by Jonathan Kehayias  first.

For Page Life Expectancy refer to Page Life Expectancy isn’t what you think… by Paul Randall.

An excellent reference :

How much memory does my SQL Server actually need? - by Jonathan Kehayias