For all things hardware regarding SQL Server, you can't do much better than Glen Berry. Check out his min/max memory settings post: http://www.sqlservercentral.com/blogs/glennberry/archive/2009/10/29/suggested-max-memory-settings-for-sql-server-2005_2F00_2008.aspx

Don't set the max SQL Server can use more than the server has otherwise a runaway query/process could choke the box out by starving the OS of memory which would possibly not let you remote in and fix the issue (while I'm at it, make sure you enable DAC connections on your servers as safeguard).

Another great resource is Brent Ozar, he has tons of performance tuning tips/articles you should read through: http://www.brentozar.com/sql/sql-server-performance-tuning/[/url]