Rather than playing with memory allocation the real way is to simply add more memory. Any thing above 12 GB seem to run circles around lessor amounts. I have never tweaked the preferences. I have seen Premiere use in excess of 9 GB and that probably does not include trasient peaks of much more.
8 GB is enough for CS5, it's not ample but enough... I'm sure there is a way to fix the situation I'm in.
Memory management in CS5 is clever enough to use an amount of memory that minimizes the use of the page file and maximizes buffers, However that depends on the material in the project, the effects and transitions used, the export format and the hardware in use.
There is no sense in using large amounts of memory, when other components cause waiting before the contents of memory can be offloaded. Just as it makes no sense to use a Ferrari in traffic jams.