The maximum amount of memory that your system can use is limited by two factors — the first is the maximum that your motherboard can handle, and the second is the maximum amount of memory that your operating system (OS) can accept.
For instance, when you install 4GB of memory in a system running the 32-bit version of Windows it will only see (and utilise) somewhere between 3GB to 3.5GB.
This doesn't mean there is a problem with your memory, or your system. The 32-bit versions of Windows XP/Vista/7/8/10 allow for 4GB of memory to be addressed, but other system limitations mean that there is less memory actually available for use by the Operating System.
What happens is that some of the addressable memory (regardless of how much you have physically installed) is reserved for use by some of the devices that you are using, such as a graphics card, PCI card, or other peripheral, so it's unavailable for use as normal main memory. Some computers with integrated video adapters also use part of the system memory for video memory.
So how much memory is reserved by these peripherals? Integrated video cards without their own video memory can use anywhere from 16MB to 256MB or 512MB of system memory. Even video cards with dedicated onboard memory can still consume large portions of the 3-4GB memory space. For example - an older 256MB graphics card will require 256mb of the 3-4GB memory space be reserved. At the other extreme, two new GeForce 7950 1GB graphics installed in SLI mode will require almost 2GB of system memory in the 2-4GB space be reserved. So if you purchased 4GB of system memory with these particular video cards, little more than 2GB of this will be available to your OS.
The maximum memory limitation varies by operating system; for instance, the 4GB memory limitation doesn't exist in 64-bit versions of Windows or in MacOS 10.6 (Snow Leopard) or later versions, which includes full 64-bit support.
The maximum memory possible for current Microsoft® Windows OS include:
Windows 10 32-bit: 4GB
Windows 10 Home 64-bit: 128GB
Windows 10 Pro 64-bit: 512GB
Windows 8 32-bit: 4GB
Windows 8 64-bit: 192GB
Windows 8 Pro/Enterprise 64-bit: 512GB
Windows 7 Starter: 2GB
Windows 7 32-bit (all other versions): 4GB
Windows 7 Home Basic 64-bit: 8GB
Windows 7 Home Premium 64-bit: 16GB
Windows 7 Professional/Enterprise/Ultimate 64-bit: 192GB
Windows XP 32-bit: 4GB
Windows XP 64-bit: 128GB
The maximum memory possible for current versions of MacOS include:
Mac OS X Server 1.0: 1GB
Mac OS X Server 1.2: 1.5GB
Mac OS X Server 10.0.x : 3GB
Mac OSX Versions up to 10.5 Leopard: 4GB
MacOS 10.6 Snow Leopard and above: 128GB (limited by hardware)