Is there any way to change out the logic board?

There are a couple of functions I wanted to do with my computer but can't because of the 'outdated' hardware. An example is native AirPlay mirroring. I know there are workarounds but have tried them all and they never work as well as native (which I have tried using my mum's Macbook Pro and works much better). I'm looking to upgrade a bunch of internals in my computer. I don't see a need to upgrade the whole computer, especially since getting the specs I want by buying a brand new computer would probably be more expensive. So I wanted to upgrade parts of the internals. I was hoping to get a Sandy Bridge Intel Processor, so AirPlay would work natively, at least. Is this at all possible?

Also, I was curious, my computer can currently only take a max of 8GB of RAM at 1067 MHz DDR3. I was wondering whether it is possible to change this to take more. And I similar case with the hard drive. I know right now it can only run 3GB/s, can I change my computer so the hard drive can run faster. And lastly, how easy would it be do achieve such things? I'm asking mainly for curiosity, but if it's all doable and easily so, I would love to do it.

My computer is already 4 years old and I want to keep it going for another 4+ years. I'll be using my computer very intensively for a lot of tasks too. I'm going to open it up to change the battery and was hoping to change a few more specs. Just want to know what I can do.