For a given workload, the more RAM you have, the less pagefile that will be required. But Windows has no idea what your workload is going to be – but it can make a reasonable guess. In general, the memory demands placed on a computer will be in proportion to RAM size. Playing a modern game with only 128MB of RAM isn’t going to provide a satisfying experience no matter what size the pagefile. A large pagefile would be a waste as performance will become unacceptable long before it is filled. A system with 4GB of RAM will be suitable for modern games, video editing, and more. This means a great deal of data will be handled, some of which will be only rarely accessed, if at all. The pagefle was designed to handle this, leaving the precious RAM for more important uses.

Windows designers learned very early that a pagefile about 1.5 x RAM size would be necessary to make optimum use of RAM, whatever it’s size. If you have 128MB of RAM and are pushing it hard, that may not be enough. If you have 4GB and use it only for light duties that will more than needed. But more than needed does not mean too much. A larger than necessary pagefile will not encourage excessive use and it will not produce fragmentation.

That was the reasoning behind the 1.5 x RAM rule. It won’t be suitable for everyone but it will usually be close.

Bottom line: unless you have a very good reason, and you understand what you are doing, leave pagefile settings on default.

Source: “Inside Microsoft Windows 2000″. Heavy reading but far more accurate than you will find anywhere else.

Larry Miller
Microsoft MCSA

….still what I said applies well for todays 1Gig and up machines still suing XP, and probably with Vista since the way to allocate data among them is practically the same.