It pains me that information for memory isn't readilly available.
The Atom is not alone in this confusion heck macbooks had this issue for some years too.
The problem started with internal memory controllers supporting 32 bit data interfacing.
Microsoft also added to the issue when they required limiting the amount of memory that netwbook could support if they wanted to use windows 7 and crippled windows 7 starter to also have a low memory limit...1GB, or full expanded 2GB.worse yet some have only one slot.
Some of the netbooks have had hackers perform some tweaks to get upto 4GB memory.
Some can have windows 7 64 bit installed and some can even have the memory increased to 6 or 8 GB under windows 7 64.
But that history all said it still comes down to search the internet for a particular netbook manufacturers model number and see what offers have found possible.
out of the box, netbooks have been crippled by the OS manufacturers and the Hardware manufacturers.
Newer dual core Atoms are beginning to show up in hardware able to naitively support 4, 6 or 8 gb. so do your research before buying.