The quality of memory that you use for building or upgrading a computer is very important indeed. The wrong stuff can at best slow it down, at worst can cause the machine to crash regularly or fail completely. What you need is to find a reliable make....
In all my research so far, the best quality memory seems to come from a big manufacturer called Micron. They make the stuff themselves, and have very high standards. The good news for you and me is that they sell their memory direct to the public at cheap prices via their website. Micron market this memory through their brandname, Crucial™.
See how cheap a speed boosting Crucial™ Memory upgrade could be for your machine...