From the Docs
1964 0.85
Well, leaving this off will only allocate 4MB for RDRAM, while turning on the expansion pak will allocate 8MB. Majora's Mask, for instance, requires it on.
1964 1.1
The Expansion Pak is a 4 MegaByte RDRAM pak that can be installed into the N64. With an Expansion Pak installed, the N64 has a total of 8 MegaBytes of available RAM. Some games such as Donkey Kong 64 require the Expansion Pak to be set to Yes, which means it is installed. Some other games might get better performance or have more game options with Expansion Pak. The default setting is No.