Here's a list of how much RAM the various Windows versions and editions support (as of Nov 2004):
Windows NT 4.0: 4 GB
Windows 2000 Professional: 4 GB
Windows 2000 Standard Server: 4 GB
Windows 2000 Advanced Server: 8GB
Windows 2000 Datacenter Server: 32GB
Windows XP Professional: 4 GB
Windows Server 2003 Web Edition: 2 GB
Windows Server 2003 Standard Edition: 4 GB
Windows Server 2003 Enterprise Edition: 32 GB
Windows Server 2003 Datacenter Edition: 64 GB
Pagefile
RAM is a limited resource, whereas virtual memory is, for most practical purposes, unlimited. There can be a large number of processes each with its own 2 GB of private virtual address space. When the memory in use by all the existing processes exceeds the amount of RAM available, the operating system will move pages (4 KB pieces) of one or more virtual address spaces to the computer’s hard disk, thus freeing that RAM frame for other uses. In Windows systems, these “paged out” pages are stored in one or more files called pagefile.sys in the root of a partition. There can be one such file in each disk partition. The location and size of the page file is configured in SystemProperties, Advanced, Performance (click the Settings button).
A frequently asked question is how big should I make the pagefile? There is no single answer to this question, because it depends on the amount of installed RAM and how much virtual memory that workload requires. If there is no other information available, the normal recommendation of 1.5 times the amount of RAM in the computer is a good place to start. On server systems, a common objective is to have enough RAM so that there is never a shortage and the pagefile is essentially, not used. On these systems, having a really large pagefile may serve no useful purpose. On the other hand, disk space is usually plentiful, so having a large pagefile (e.g. 1.5 times the installed RAM) does not cause a problem and eliminates the need to fuss over how large to make it.
Performance, Architectural Limits and RAM
On any computer system, as load (number of users, amount of work being done) increases, performance (how long it takes to do each task) will decrease, but in a non linear fashion. Any increase in load (demand) beyond a certain point will result in a dramatic decrease in performance. This means that some resource is in critically short supply and has become a bottleneck.
At some point, the resource in critical short supply can not be increased. This means an architectural limit has been reached. Some commonly reported architectural limits in Windows include:
1. 2 GB of shared virtual address space for the system
2. 2 GB of private virtual address space per process
3. 660 MB System PTE storage
4. 470 MB paged pool storage
5. 256 MB nonpaged pool storage
The above applies to Windows 2003 Server specifically (from Knowledgebase article 294418), but also apply to Windows XP and Windows 2000.