On a dedicated database server, how much memory to reserve for the OS? Assuming you have a dedicated server explicitly for database functions -- how much memory should you reserve for the operating system?

I realize this will probably vary somewhat depending on the specific OS, the specific database software, and so forth. But, as memory is so important to database performance, I want the database to have the maximum reasonable level of memory, without starving the host OS.

So


what's a good rule of thumb to start with?
what counters or performance indicators should we look at to determine if we've gone too far and the host OS is being starved somehow by the database?
Assuming Windows and SQL Server...

There are two schools of thought.  


Leave 2-4 Gigs for Windows (depending on what is installed besides SQL Server).
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.