More than the Hardware issues mentioned here in various replies to your question , you should also know that there are Operating Systems limits as well. Now, if you put in 10 GB of RAM into a Computer with an Operating system that does not have the capacity to use that much memory or the need for that much memory then you will be wasting your money buying all that much RAM. Both the Operating System installed on the computer and the Motherboard should be able to support the RAM that will be installed. There is a limit to the speed that can be increased by increasing the amount of RAM within a computer. If this was not the case then one could make a 486 Processor run at the speed of Pentium IV merely by adding a whole lot of RAM.
You also need to know what you will need so much RAM for. If you are a high end gamer, if you use lots of Graphics, if you are into Video / .mp3 editing etc then it will be great to have lots of RAM. But if you are a normal user with normal needs ( email, surfing the Web, downloading useful files etc., burning CDs ) from a Computer then I will suggest that you increase the amount of RAM as much as you can afford. This should not be seen as a way to increase the speed of the Computer.