All depends on what the motherboard can take, and what the rest of the system can use. What is the motherboard, and what was the original system? Faster RAM is always good, but if it's not the slowest thing in the system, you won't see much gain until it is. If you need more info to decide, go to a site like Crucial.com, and see if they explain it very well. If the price is right, by faster RAM, but if it's too much, and the speed gain is negligable, that won't help you much.
Ed