Some people say 1.5x physical RAM, others say 2.5x, others add to one of those, not to exceed 350MB pagefile due it taking too long to read the file, of course that was a few years ago. I say to start with 1.5x and work from there to find the best setting for your needs.
Personally I have 1.5GB physical RAM and a 348MB pagefile. I do a lot of video editing and 3D rendering and haven't had any problems but I have had my free RAM at a very low level, though I rarely use more than around 20MB of my pagefile other than certain apps which insist upon using it just because they feel the need to do so.
I found that a larger pagefile, in my case, just increased the time it takes to reboot my machine when the need arises.