Hi Scania

This is extremely debatable. Mr Fozzer has Win98SE and put an extra 256mb RAM in (taking the total to 768mb) and has apparently noticed a big increase in performance.

I myself have come to the conclusion that whether you can add more than 512mb RAM to those OSs depends completely on the system.

For example, two of my friends have Windows 98. They both put their RAM from 512mb to 768mb and one noticed a 5-7fps performance increase in FS2002, the other found that it slowed their PC down so much it was almost unusable.

Jon 8)

SYMPTOMS
If a computer that is running any of the versions of Windows that are listed above contains more than 512 megabytes (for example, 768 megabytes) of physical memory (RAM), you may experience one or more of the following symptoms:
You may be unable to open an MS-DOS session (or command prompt) while Windows is running. Attempts to do so may generate the following error message:

There is not enough memory available to run this program.
Quit one or more programs, and then try again.
The computer may stop responding (hang) while Windows is starting, or halt and display the following error message:

Insufficient memory to initialize windows. Quit one or more memory-resident programs or remove unnecessary utilities from your Config.sys and Autoexec.bat files, and restart your computer.
CAUSE
The Windows 32-bit protected-mode cache driver (Vcache) determines the maximum cache size based on the amount of RAM that is present when Windows starts. Vcache then reserves enough memory addresses to permit it to access a cache of the maximum size so that it can increase the cache to that size if needed. These addresses are allocated in a range of virtual addresses from 0xC0000000 through 0xFFFFFFFF (3 to 4 gigabytes) known as the system arena.

On computers with large amounts of RAM, the maximum cache size can be large enough that Vcache consumes all of the addresses in the system arena, leaving no virtual memory addresses available for other functions such as opening an MS-DOS prompt (creating a new virtual machine).
WORKAROUND
To work around this problem, use one of the following methods:
Use the MaxFileCache setting in the System.ini file to reduce the maximum amount of memory that Vcache uses to 512 megabytes (524,288 KB) or less. For additional information about how to use the MaxFileCache setting, click the article number below to view the article in the Microsoft Knowledge Base:
108079 32-Bit File Access Maximum Cache Size

Use the System Configuration utility to limit the amount of memory that Windows uses to 512 megabytes (MB) or less.For additional information about how to use the System Configuration utility, click the article number below to view the article in the Microsoft Knowledge Base:
181966 System Configuration Utility Advanced Troubleshooting Settings

Reduce the amount of memory that is installed in your computer to 512 MB or less.
STATUS
Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.
MORE INFORMATION
Vcache is limited internally to a maximum cache size of 800 MB.

This problem may occur more readily with Advanced Graphics Port (AGP) video adapters because the AGP aperture is also mapped to addresses in the system arena. For example, if Vcache is using a maximum cache size of 800 MB and an AGP video adapter has a 128-MB aperture mapped, there is very little address space remaining for the other system code and data that must occupy this range of virtual addresses.