SQL Server is designed to manage memory needs dynamically, and it does an excellent job in most cases. Using dynamic memory allocation, SQL Server can add memory to handle incoming queries, release memory for another application you are starting, or reserve memory for possible needs. The default memory settings are the following:
Dynamically configure SQL Server memory
Minimum memory allocation set at 0 MB
Maximum memory allocation set to allow SQL Server to use virtual memory on disk as well as physical RAM
No memory reserved specifically for SQL Server
Address Windowing Extensions (AWE) not enabled
Minimum memory for query execution set at 1024 KB
You can change these settings, but you need to be careful about allocating too little ...