How can i determine real memory needed for SQL server?

Hi,
I´m currently using a SQL Server on top of VMWARE Server with Win 2008 64 bits, with 4 GB of RAM. I´ve assigned 3 GB to SQL Server and the rest left it to OS management. But I always keep getting the impression that the box needs extra memory. Currently we´re running primarily batch processes. Are there any best practices out there regarding SQL?
Regards.
