Oh forget this whole PC ram in the processor stuff because that really isn't even called ram thats Cashe not ram. So get that striaght. And yes when you by a motherboard you have to buy ram for it. There are all kind of brands. Me i go with GSkill because they are very reliable. have bought dozens of sticks of ram of this brand and out of like 50 sicks one went bad but they have lifetime warranties.
When you buy a motherboard it will tell what kind of ram it suppots. So like DDR3-1866. The 1866 is the speed of the fastest ram it can take. It can take any speed or DDR3 that is that speed or lower. Now there is DDR DDR2 and DDR3. Any PC from the past like 4 years will most likely be DDR3. There are a lot of different brands. OCZ, Crucial, Kingston, GSKill, Corsair ect. Again i'm a big GSkill fan just because their product seems to work the longest and be the most compatible.