I'm having a hard time figuring out whether or not a certain laptop/computer's RAM can be upgraded or not. Is there a rule of thumb that determines how much max RAM one could add to a system without looking it up via external websites? A little bit of a background information: I work in computer sales at a computer electronics store, so it is virtually impossible for me to install any sort of software that would detect computer specs, and I get a lot of customers who wonder what laptop/desktop RAM upgrades usually are. Is there a certain rule that adding more RAM entails? Does it make a difference if it's a 32-bit or 64-bit machine? OS?