Max RAM Amount that Your Operating System Supports
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Windows 8 64 bit Enterprise,Professional : 512 GB
Windows 8 64 bit :128 GB
Windows 8 32 bit : 4 GB
Windows 7 64 bit Ultimate, Enterprise & Professional :192 GB 
Windows 7 64 bit Home Premium:16 GB
Windows 7 64 bit Home Basic : 8 GB
Windows 7 32 bit (all editions except Starter) :4 GB
Windows 7 32 bit Starter : 2 GB
Windows Vista 64 bit Ultimate, Enterprise & Business: 128 GB
Windows Vista 64 bit Home Premium :16GB
Windows Vista 64 bit Home Basic : 8 GB
Windows Vista 32 bit (all editions except Starter) : 4 GB
Windows 7 32 bit Starter :1 GB
Windows XP 64 bit : 128 GB
Windows XP 32 bit : 4 GB
Mac OS X 64 bit :96 GB
Linux 64 bit :1 TB
Linux 32 bit :4 GB
Linux 32 bit (with PAE) :64 GB