In Microsoft SQL Server 2008 the Standard, Enterprise, Developer and Web editions can use whatever the Operating Systems maximum allowed memory is.
Windows Server 2003
Windows Server 2003 Standard Edition can use a maximum of 4 GB.
Microsoft Windows Server 2003 Enterprise Edition can use a maximum of 32 GB for 32-bit (x86) machines and 64 GB for 64-bit Itanium machines.
Windows Server 2008
Windows Server 2008 Standard Edition can use a maximum of 4 GB.
Microsoft Windows Server 2008 Enterprise Edition can use a maximum of 64 GB for 32-bit (x86) machines and 2 TB for 64-bit Itanium machines.
The Workgroup edition can also use the Operating Systems maximum on 32-Bit systems and 4 GB on 64-bit systems.
The Express and Express with Advanced Services can use a maximum of 1 GB.