generally i think if you have <512mb's ram make the pf 1.5-2x the amount of ram you have. 512 and > i usually kepp mine about 768 (1.5x the amoount of ram). if you have greater than 1gb i'd just match the pf to the ram, but probably wouldn't go over 1gb. you (usually) really don't need a huge massive >1gb pf, it's overkill and a waste of space. also, i would set it to one value, ie. fix it to say 768mb, 768 initial size and 768 maximum size. i would set your's to 512 probably... speed of ram doesn't matter, only amount.
people doing massive video editing etc shouldn't have less than 1gb of ram anyways, and i would apply the above to that as well.