I have a Dell GX520 optiplex model (mini tower), and I want to upgrade its RAM. I currently have 1GB RAM and I want to upgrade to 2GB. All I know about RAM is its capacity, and I've heard about speeds lately like 400mhz, 533mhz, 800mhz, 1333mhz. I want to know what are the differences among them and can my PC support all those types of speeds or not? And what is the most suitable one for my Dell PC, to avoid any problems or limitations.