PS CS4 has a 64bit native version. So it can use gobs of ram.
The rest of the suite (inclding Pr and AE is nto 64bit native. They are 32 bit apps. So they cant just grab all the mem they want. However; they have been designed (in CS4) to spawn multiple processes for certain functions so each process can use mem; collectively being more effecient on a machine with lots of ram.
Bottom line; in XP you cant throw more mem at the problem. In Vista64 you can; and it makes a big difference in performance; even though Pr and AE are 32 bit apps; they benefit from more ram..
premiere is still a 32 bit program but it has been optimized for 64 bit os. see link above.
edit...sorry Curt, you beat me to the punch.
Each 32 bit application can only use 4 GB max, but PR has been designed to take avdvantage of more memory if installed. During certain operations other processes are used that use their own memory space. The net effect is that PR runs faster when more memory is installed on a 64 bit OS.