I suppose this is pretty straight forward but I was needing to add some memory and I was wondering what I needed to know. I have 1 Gig but as I've added a lot of software lately my machine has slowed so I'm wanting to add more memory. I think the 1 Gig I have is a total from two boards so do I have to add in the same increment? In other words if what I have is two 500 Mb boards do I have to continue with that denomination (for lack of a better term)? Can a person mix and match memory boards as far as size is concerned?