A computer is designed with a maximum amount of memory. Our configuration guides with our memory upgrades has a known maximum amount of memory a system can recognise. There are always exceptions.
There are also OS (Operating System) memory limitations.
The most popular ones are:
Windows 95: 1Gigabyte
Windows 98: 1Gigabyte
Windows 98SE: 1Gigabyte
Windows ME: 1.5 Gigabytes
Windows NT: 4 Gigabytes
Windows 2000 Professional: 4 Gigabytes
Windows XP: 4 Gigabytes
Windows XP Professional 64-Bit: 128 Gigabytes
Windows XP Vista 32-Bit: 4 Gigabytes
Windows XP Vista 64-Bit: 128 Gigabytes
Windows 7 64-Bit: 192 Gigabytes
OS X: 32 Gigabytes
OS 9.x: 1.5 Gigabytes
Linux: 64 Gigabytes