Can you add RAM?

[Jango]

I want to upgrade from 2X 1GB DDR2 667Mhz RAM to 3GB

Can I just add another 1GB of the same RAM in another slot?

 

[rxmac]

Assuming you have spare memory slots you can, but it might be better to add another 2 sticks to keep dual channel, as it gives slightly better performance, so either 2x 512mb sticks or even 2x 1gb sticks for 4gb total (though you might only see 3.5gb dependant on operating system and other hardware).