No disrespect to e-buyer (I use them a lot), but when it comes to memory, there really is only one company to go to click here
You can use their on-line memory finder to guarantee you will get the correct memory for your computer and there wont be much difference in the price from what you've already seen.
As it stands, you've asked this question under the Absolute beginners forum, so are you sure you have the right memory? It's critical it is!
As regards fitting, it will have an offset bar, which stops you fitting it incorrectly. Be aware of "static electricity", as it's easy to damage RAM. Again, use the resources on Crucial to see exactly how to go about fitting your new memory.