The memory decisions are a little more clearer once you’ve sorted out you processer and motherboard, they will certainly give you most of the answers when it comes to your own system. Once you have your motherboard selected you will then know from its specifications what type of memory to get, again there are several types with different amounts of pins along the edge, 240 and 184 are two of the most popular requirements along with the type of memory and the speed requirements, you should get the fastest that your motherboard supports as you don’t want to be slowing your system down because of the memory which is one of the cheaper components.
You will also need to decide how much memory you want. I would consider 2 Gigabyte’s to be a minimum these days with 3 or even 4 being a better long term solution. The reputable names to look out for when it comes to buying memory are Corsair, Crucial, Kingston and Patriot. Crucial also have a useful utility on their website that can help you identify the type of memory you need.