The paging file, in general, should be approximately 1.5 times as much RAM as you have. For a system with 512 MB of RAM this would mean that your paging file should be 768 MB. This is generally how its setup. You can let Windows automatically set the paging file size for you, but some people recommend setting it manually. This is so that you can set the minimum and maximum paging file size to be the same thing to reduce fragmentation on your drive. This can be set by right-clicking My Computer, going to 'Advanced', then 'Performance Options'. Paging file info is on the bottom of that screen.
HTH,
Dr Tree