See Chapter 4 of http://www.red-gate.com/community/books/accidental-dba for a lot of discussion of memory and some guidelines around the setting of this value.
The setting takes effect immediately, no restart is required.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability