Before you buy anything, open up the start menu -> right click on "computer" in the right-hand panel and click "properties". A window should pop up and one of the listed items will be "Installed memory (RAM)" and tell us what that says. Unless your computer is SERIOUSLY lacking in memory (like, you have a total of 1GB) I don't think investing in RAM will be worth your while. In my experience, it provides the least benefit, if any at all, for casual users. RAM doesn't necessarily speed up a system, it just gives your system more places to put your currently running programs for faster access.

If you do decide to upgrade your RAM, you'll need to open up your laptop and see how your RAM is configured. Computers generally have 2 RAM slots, and if your computer maker is smart, they'll fill both slots (e.g. if you have a total of 4GB of ram, you'll have 2x 2GB sticks installed). installing two sticks of the same size increases RAM speed.

I stand by what I said - installing an SSD will give you the best performance. You can get a seriously fast 250GB drive for under $70 now.