Ram upgrade

I think, you should go in for a 2 GB or a 4 GB module, depending on your requirements, availability and of course budget of the RAM sticks. Since, it's a DDR2 ram, it can be either 533, 667 or an 800 MHz one. Use utilities like Sisoft Sandra, etc, to precisely check the actual frequency of your RAM, and you could try to purchase one either from a shop near your location or you could try Flipkart. They have started selling Computer components. You could look at brands like Kingston, Transcend and Corsair (though a bit expensive than others).

With an atom based machine, 2gb ram makes sense. Ram higher than that with atom is just wasted ram (the processor needs to be equally adept to utilize that much ram).

Atom will not take the advantage 4gb ram. If it would have, Intel would have probably used it as a marketing gimmick

Cheers!