I think you may be misunderstanding how RAM works. RAM is a chip which is usually in the same package as the CPU, and which communicates with it at a very high speed (about ten times as fast as Ethernet). The N900 has 256 MB of RAM. The Raspberry Pi also has 256 MB of RAM. So theoretically, you could network the two together and have a device with 512 MB of RAM. But by doing so over a network, you lose the key advantage of RAM, which is the access speed. You would get better results by using some of the flash memory as RAM. This is called "swap space", and both Maemo and Android do this already (you can find some posts further back on how to optimize Nitdroid by changing the swap parameters.)

So, to answer your question: It is theoretically possible. But it is also useless.