Different Windows editions have had different upper maximum RAM limits. 64-bit Windows support different amounts of memory as a SKU-differentiating feature, with the low end being 512MB for Windows XP Starter to 128GB for Vista Ultimate and 192GB for Windows 7 Ultimate.
All 32-bit Windows client SKUs, however, including Windows Vista, Windows XP and Windows 2000 Professional, support a maximum of 4GB of physical memory. 4GB is the highest physical address accessible with the standard x86 memory management mode. Hardware Specifications for Windows 7 lays down the minimum memory (RAM) requirements.
So, while the maximum RAM limit for 32-bit Windows 7 editions is 4GB, when it comes to the 64-bit editions, the amount of memory that the OS can address depends on which edition you are running.
Here are the upper RAM limits for the different editions of Windows 7:
Starter : 2GB
Home Basic : 8GB
Home Premium : 16GB
Enterprise : 192GB
Ultimate : 192GB
These limits are similar to those for Vista editions, except that Vista Enterprise and Vista Ultimate have had their upper limits raised from 128GB to 192GB.