|Go to www.crucial.com and use their scanner. It will tell you how much memory, type of memory you have and what your options are to add more. |
As noted, if you have a 32bit version of Windows, do not bother. You need a 64bit version to use more memory than you have.
If you do just basic computing, homework, internet (including facebook, skype, etc), email, you will not see much change in your computing with over 4GB of memory. If you use Photoshop, Autocad, Video processing software, Excel files with near 65,000 lines or more, or other software that uses massive amounts of memory, then you will see an immediate improvement.
You have to be a little bit crazy to keep you from going insane.