Maximum memory supported by operating system

Memory upgrades for Windows 8 Editions
Windows 8 has a minimum memory requirement of 1 GB RAM (32-bit) or 2 GB RAM (64-bit). Information about the maximum amount of memory is not available at this time.
Memory upgrades for Windows 7 Editions

Windows 7 has a minimum memory requirement of 1 GB RAM (32-bit) or 2 GB RAM (64-bit). The maximum amount of memory depends on the edition used:

Edition of Windows 7	Maximum addressable memory
Starter (32-bit or 64-bit)	2 GB
Any 32-bit version of 7 (except Starter)	4 GB (approximately 3.3 GB available for use)
Home Basic 64-bit	8 GB
Home Premium 64-bit	16 GB
Enterprise 64-bit	192 GB
Business 64-bit	192 GB
Ultimate 64-bit	192 GB

Memory upgrades for Windows Vista Editions

Windows Vista comes in a variety of editions. All versions of Vista have a minimum memory requirement of 512 MB (1 GB to take advantage of certain premium features, such as Aero graphics). The maximum amount of memory depends on the edition used:

Edition of Windows Vista	Maximum addressable memory
Starter (32-bit)	1 GB
Any 32-bit version of Vista	4 GB (approximately 3.3 GB available for use)
Home Basic 64-bit	8 GB
Home Premium 64-bit	16 GB
Business 64-bit	128 GB
Ultimate 64-bit	128 GB

Memory upgrades for Windows XP

Windows XP comes in 32-bit or 64-bit version. The maximum amount of memory depends on the version used:

Version of Windows XP	Maximum addressable memory
32-bit	4 GB (approximately 3.3 GB available for use)
64-bit	128 GB