For most people I would think the standard 1GB would be plenty. Why do you want to upgrade to more memory? Memory upgrades are not supported. See http://www.readynas.com/hcl
If you examine the memory in the NAS you will be able to tell the spec. It would have a single memory module. I don't know what the limit is.
If you do do a memory upgrade be sure to run the "memory test" using the Ultra 2 Boot Menu
before trusting the memory.