I want to add RAM memory. A scan of my computer system told me that it would support up to 3 GB of RAM. I now have 1 GB. To upgrade to 2 GB, do I have to buy a matching pair of RAM thingies, or can I keep the 1 GB RAM thingie that's in there, and add another one, so long as the new one is compatible??????? (It's an HP Pavilion running Vista 32-bit.)