So, to add to Begginnerdev's point, different motherboards work with memory at different speeds. So, you will want to make sure that the memory you are buying is compatible with your particular motherboard. Crucial has an awesome memory scanner that I would recommend checking out if you are unsure. You can also just check the specs on your particular motherboard.
Your best thing to do, is to use Crucial.com advisor, and it will tell you the correct RAM for your machine. Then shop around. I will tell you up front, that RAM prices have gone up. No matter where you shop, all are going to be around the same price.
At least with Crucial, it is a no hassle return if they send the wrong kind, or it goes bad at any time. I only purchase all my SSD's and RAM through them and no one else.