There are 2 methods stated in the knowledge base:
1. Add the entry in about:config
2. Add the entry in user.pref
Which one did you use?
I personally use option #2. Then create all the appropriate folders & subfolders, including the "cache" subfolder. However, in the path name listed in user.js, the "cache" subfolder name is not entered.
Example: I want to move the cache to E:\Personal_Info\Mozilla_Cache\FF4\Cache
Then, the entry in user.js will be
user_pref("browser.cache.disk.parent_directory", "E:\\Personal_Info\\Mozilla_Cache\\FF4");
I also notice that if there is a space in the path name, sometimes FF is not too happy about that.
Other resource on moving cache to RAM Disk:
»
www.ghacks.net/2007/12/1 ··· ecurity/(edit: Corrected "user.pref" to "user.js")