First off, what is RAM? Random Access Memory is the workspace your computer uses to do everything it does.
RAM is often confused with the Hard Drive. The hard drive is storage, whereas RAM is where the files that have been pulled out of storage are worked with. Just like in an office where you pull a file out of the file cabinet and take it to your desk to work with it.
So, how much do I need? This depends on three things: the version of Windows you have; the age of your computers motherboard; and how much money you have.
If you have Windows XP, the most you can have is 4GB, regardless of the other factors.
If you have a really old computer with Windows XP, then the maximum is between 2GB and 4; a little investigation is required to find the exact amount you can have.
If you have Windows Vista, you have to look in “Computer Properties” to see if you have 32-bit or 64-bit; the first is restricted to 4GB, the second may or may not be.
If you have Windows 7, and you bought it at a store, then you have 64-bit and you are only restricted by your budget and the motherboards circuitry.
How much do you need? For Windows XP and 7, 1GB will do, but more is better.
For Windows Vista, 3GB; and more will make your life easier.
There you go.