How easy is it to buy upgrade/replacement parts for a Mac Pro? I'm considering purchasing a Mac Pro in order to save money on upgrades in the future. I know how to build my own PC from scratch and I was wondering if I could use the same principles if were to buy Apple supported hardware. Do not get this confused with a Hackintosh, that's not what I'm striving to achieve.

Would I be able to upgrade the logic board? and CPU? Or the Power Supply?

Do you have experience with this and how did it work out for you?
Apple uses custom logic boards, but I've heard of people upgrading the CPU if it uses the same socket type. The power supply on my 2008 Mac Pro would be hard to get to for a replacement. The key upgrade options are new hard drives, more memory, and better video cards. I've used those three to keep my '08 Mac Pro running fast enough to more than meet all of my current needs.