Assuming Windows and SQL Server...

There are two schools of thought.

1. Leave 2-4 Gigs for Windows (depending on what is installed besides SQL Server).
2. Leave 10% of your available memory free. As you get over 64 Gigs this gets to be a crazy large amount of money to leave for the OS, which it probably won't need.

Personally I'm in the first group. Windows usually just needs 2-4 gigs, sometimes up to 6.

Assuming Windows and SQL Server...

There are two schools of thought.

1. Leave 2-4 Gigs for Windows (depending on what is installed besides SQL Server).
2. Leave 10% of your available memory free. As you get over 64 Gigs this gets to be a crazy large amount of memory to leave for the OS, which it probably won't need.

Personally I'm in the first group. Windows usually just needs 2-4 gigs, sometimes up to 6.