the RAM the system motherboard is designed to use.
I suspect you actually mean what is the best setting for VIRTUAL MEMORY (the Windows page file) and the old rule of thumb about it being set to one and a half times the physical RAM still holds as a "rule of thumb" but is best adjusted by careful observation of committed memory in the task manager.
In general, with Windows XP, you can get good results by leaving things with the default setting and letting Windows manage its own paging file dynamically.
What is the best RAM for a windows xp when you can change the initail and maximum size? I heard that I should change it to 4096. Is this best?