It depends - how much memory does the server have? Is this a single instance server - in other words, are there other SQL Server instances running on this server?
Is this a 32-bit system, or 64-bit?
If 32-bit, do you have PAE enabled?
Do you have the /3GB switch set in boot.ini?
Are you running anything else on the server? For example, is it a DC - do you run SSIS packages, etc...
All of the above will help determine if you have the memory configuration setup correctly.