George,
It really depends on what the customer designed applications are doing and the number of users. So it's impossible for me to say. Personally, I would never recommend a production system less than 12 GB and 16 GB is more common. Mind you, the OS also needs memory, 2 GB for 64-bit OS I believe. There is the SAP sizing guide you can use. Although, memory is fairly inexpensive compared to taking a performance hit later.
-Kevin
