Everybody will tell you something different, although generally the ideal way is to test your system at full load (approximately what you'd expect to be the most amount of stuff you'd run simultaneously) and gauge it based off those numbers. Some applications or games might complain if you don't have a certain amount, though.
Personally - and I have 4GB of RAM - I spread my pagefile over 3 HDDs, each with 1536MB (512x3) min/max (4608 total).