Just a bit of my history here so you have a clear picture of my experience (and ONLY my experience).
I have a Dell Inspiron 6400 laptop that was running Windows XP on 2GB of DDR2 RAM. After installing SP3, I felt that the system was too sluggish, so I purchased and installed a second 2GB DDR2 RAM module. After that, the system ran perfectly fine for a couple of years. The system was never intended for high-end gaming. It's main purpose was more for software development and testing and as I said, it ran fine for a couple of years.
One day, I powered it up and ran into a memory fault error. On the Dell laptops, that more of a flashing lights code, but some research told me that a memory module had an error. Testing both memory chips in the first slot, both worked just fine. Testing both, one in each slot gave me the error again. I then deemed by second memory slot had failed (probably due to heat).
By this time, I was mostly done with my software development and testing and was really only using the system for finances and as an emulator. It ran just fine for those uses for another couple of years until, finally, the primary RAM slot failed. Whether this was due to heat of just an eventual motherboard failure, I don't know. I suspect the first slot failure did enough damage to make the second slot failure, inevitable, but I can't be sure.
I'm not saying this WILL happen to you. I'm not even saying this is typical of adding RAM to laptops. I'm just saying this was my experience and to watch out for it.
-Wolf sends