It seems to be a personal thing, and there are no hard facts supporting any particular size. However, I think it is appropriate to have approximately 300-400 total memory available to the operating system. Therefore if you have 256MB ram, set it to 100-150. With your 512, I would tinker with it and run it as small as I possibly could and still have all of my software function properly. I would personally not disable it though, there are programs that require it. Be careful, you can start a flamewar with questions about page files, some people think that they *know* what it should be. Ignore those people and do what works best for you.
BTW I have 256MB ram, and I set mine to MIN:0, MAX:100. It's extremely snappy and doesnt even use a pagefile until I start to really abuse the system. I use these settings in Windows 2000 and Windows ME.