|If you have too little memory, then adding more can be very helpful. If you already have enough, then adding more will not gain you much advantage.|
What you really need to do it tell us what computer make and model you have, how much memory you currently have, and what operating system you have (XP, W7 32bit, W7 64bit, etc.). Then also tell us what you use your computer for since Internet/email vs. Office/gaming vs. Photoshop/Autocad all require vastly different amounts of memory.
Answer these questions and then we can give you a reasonable recommendation and you can see if it fits your budget.
You have to be a little bit crazy to keep you from going insane.