Standard and Enterprise both support the OS maximum.

I'm guessing you're running on Windows 2003 Server? If so, the OS only allocates AWE memory to SQL Server on demand - different from the way it works on Windows 2000 Server where it will all be allocated at once. This is from http://msdn2.microsoft.com/en-us/library/ms178067.aspx

Use min server memory to guarantee a minimum amount of memory available to the buffer pool of an instance of SQL Server. SQL Server will not immediately allocate the amount of memory specified in min server memory on startup. However, after memory usage has reached this value due to client load, SQL Server cannot free memory from the allocated buffer pool unless the value of min server memory is reduced.

Note:

SQL Server is not guaranteed to allocate the amount of memory specified in min server memory. If the load on the server never requires allocating the amount of memory specified in min server memory, SQL Server will run with less memory.