Windows 7 Physical Memory Limits

Version Limit in 32-bit Windows Limit in 64-bit Windows
Windows 7 Ultimate 4 GB 192 GB
Windows 7 Enterprise 4 GB 192 GB
Windows 7 Professional 4 GB 192 GB
Windows 7 Premium 4 GB 16 GB
Windows 7 Basic 4 GB 8 GB
Windows 7 Starter 2 GB 2 GB


Source:
http://msdn.microsoft.com/en-us/library/aa366778