I agree that the mother board is off limits on a Mac, but there is more than just memory that you can modify. I added a maxtor hard drive (for a total of two), a Revolution 7.1 Audio card, an IO Gear USB 2 card, and AirPort card, an internal Zip drive and maxed out my memory.
This is more of the Chevy and Ford interchangeability. Both can be supped up but not with the same parts. The transmissions have different bolt patterns. Video cards have different software installed for different platforms but the edge card connections are becoming standardized.
With the coming of the Mactel you will see more, not less, of the same. I even heard that the Intel processors are socketed in the new Macs. The processor will have to have Mac programming, but soon that upgrade will be just as geeky as you want to get.