This is a really good question, because a lot of controversy has raged about this particular subject, most of it completely misinformed. With Windows 2000 Service Pack 1 and above, the best answer is to let the machine judge the size of the virtual memory file for itself. From Service Pack 1 on out, a number of under-the-hood changes were made to the memory manager that made it much more self-tuning, and the internal tunings provided by Windows are likely going to be a lot better than any guesswork on the part of the user (even if it's educated guesswork). When you set the pagefile size in the Virtual Memory window, the program will list a "Recommended" setting; the best "Initial Size" setting is this "Recommended" setting, with a "Maximum" size of twice that.
This was first published in January 2004