Limits on memory and address space vary by platform, operating system, and by whether the IMAGE_FILE_LARGE_ADDRESS_AWARE value of the LOADED_IMAGE structure and 4-gigabyte tuning (4GT) are in use.
Limits on physical memory for 32-bit platforms also depend on the Physical Address Extension (PAE), which allows 32-bit Windows systems to use more than 4 GB of physical memory.
The following table specifies the limits on physical memory for Windows 7.
To learn more about the memory & address space limits for other Windows releases like Windows Server 2008, Windows Vista, Windows XP, Windows 2000, 32-bit / 64-bit, etc visit MSDN.
Physical memory allocation and memory status in Windows 7 may interest you if you are looking for more on this!