The pagefile size is one of those ongoing arguments for which I've never found any agreement among the experts. That said, the way I size it is to first put the heaviest simultaneous workload on the computer as I would normally do in a session -- open up a browser or three, word, excel, rip a cd, encode a video whatever.

After that, open up Windows Task Manager by pressing CTL-SHIFT-ESC simultaneously and select the "Performance" tab.

Under "Commit Charge" you will have three values -- Total (the amount of physical memory & virtual memory you're currently using), Limit (the maximum physical & virtual), and Peak (the highwater mark of physical & virtual you've used this session).

I'll resize my pagefile so the Limit becomes Peak + say 20% or so as a buffer.

This is very unscientific and I may be talkin through my hat, but it seems to work for me.