They used a module, which increased it from 128Mb to 256Mb. It wouldn't go higher. http://www.tototek.com/store/index.php?main_page=product_info&products_id=116 http://assemblergames.com/l/threads/dr-v64-256-mb-neo-myth-n64-cart-upgrades.43749/
from the top of my head (and its been a while) its the same as upgrading the RAM on your pc from what I remember, has space for 2 RAM modules each come in 128Mb so you cant go above 256Mb some games would not be playable e.g. Paper Mario Ogre Battle 64: Person of Lordly Calibur Pokemon Stadium 2 Resident Evil 2 Conker's Bad Fur Day