[Do I have to buy the same size upgrade as the memory module currently installed in my computer or can I mix different sizes?]

In newer systems using SDRAM or DDR SDRAM memory, you can use modules of different densities with no problem. For example, if your computer came with a 128MB memory module, you can add a 256MB module for a total of 384MB of RAM. However, if you have a "dual-channel" system and want to take advantage of that technology, you will need to ensure that the modules in each memory slot are the same density.