Please accept, I would not wish to be rude to you, but if you don't understand "what fits into what", should you be considering upgrading your computer yourself?
You should understand, there are many more issues to consider, other than those you have already mentioned.
For example, if your existing machine is relatively old, it is possible it has an "AT" case. New motherboards require an "ATX" case. If you are considering a new generation "AMD" processor, you will require a minimum of a 300watt AMD approved power supply. Some graphics cards will not work with some processors or chipsets. The list goes on.
Please don't feel I am trying to put you off, I am simply pointing out just a few of the potential, (and expensive) pitfalls.
My suggestion is, you find a good source of information on building and/or upgrading computers, either on the web or probably a better bet, at your local library, and build up a starter knowledge base, before you attempt your goal.
There were a good series of articles in PC Adviser recently and I know you can get back copies of that magazine.
The short answer is, no one can offer sensible advise about all the variations that are available to you.
The alternative is, to post the specs. of your present machine, and perhaps someone will be able to come up with a viable upgrade for you.
I really hope this helps. It is not ment to be a negative response.