Answer:
The only possible disadvantage to adding RAM is incompatibility. However, if you do your research and get the proper type of RAM from a good source, there are only advantages: your computer will run faster and smoother; you can use more types of software; and you will be able to use newer types of hardware.
I would recommend getting as much RAM as you can afford. The newest and most you can get is often overpriced. Wait until it drops down or get the second-highest amount. In the computer world, the latest thing is always expensive and a couple of months later it will be much less in price. Lastly, don't pay for installation. RAM is one of the easiest things to install and you can find good directions on the Internet.