I am really poor in computer hardware side. I am too lazy to open up the machine and check what type of RAM (DDR/DDR2/DDR3) do my computer has and whats the Speed. These facts are really important when you really want to upgrade RAM. It should compatible with the motherboard and other Memory Cards you already use in your computer to make your upgrade effective.
One way of checking the RAM type, Model and the speed is to remove the cover of your computer and take the existing RAM module and take it to the shop or to note down the model number and the type which is printed on the module.
If you have the Motherboard manual that is another good place to check compatible RAM modules and upgrade limits. You can always refer to online manual by just Googling the model number of your computer. however, if you assemble your computer it will not have a model number you can refer to.
There is small piece of software that can be used to check more details about your motherboard and RAM. That is called cpuz. it is quite small program and you do not have to install it as it can be directly run from a external memory device.
Download cpuz version 1.50. Only thing you need to do is to double click and after couple of seconds it will show you all the details about your motherboard and the RM modules. You can use it to research a bit about it and figure out the best suite RAM module for you.