Always buy RAM from Crucial
I needed some extra RAM in my 512MB laptop to do some Java heavy lifting (Eclipse, Netbeans, JBoss, Profilers, etc). Those things can take up a lot of RAM, not leaving much for Firefox and other apps. Seeing that my laptop does not cover upgrading RAM under the warranty (very strange) unless done by authorised personnel, I went down to the store I bought my laptop from (Micro Anvika) and asked for a quote. 130 pounds is how much they wanted for a 1GB stick; in contrast I could get the same type of RAM from Crucial for 89 pounds!! That’s a saving of just over 40 pounds! I was hoping it would all go smoothly and the RAM would work because if something messed up I would be without a laptop and a warranty 
It all went perfectly of course and a month later I’ve had no problems with the RAM. It’s so simple to buy it from crucial too. They have a comprehensive database of products (computers, laptops, motherboards, etc) and the type of RAM these products take. Simply make the right selections and they tell you what to buy, even how many slots you should have free by default. If even that’s too much for you and you happen to be on a Windows PC or laptop they have an ActiveX applet that will download and try to figure out the RAM type for you. This didn’t work for my system though.
Anyway, the moral of this story is: Always buy RAM from Crucial. It is crucial that you do so