Well i myself have a MacBook. (non pro) And pretty much all thats upgradable on it is the RAM. You can get them in with bigger Hard drives, but i dont know about upgrading after you got it.. From what i know the Desktop ones are pretty much the same. More assembled units then PC's and there interchangeableness.
Ive never looked into the high end desktops from mac, mainly because they cost heaps! but thats because there around the lines of Duo Quad cores and more for your high end design sort of stuff.
Also on your overclocking stuff, Ive never been interested, because my mac has never not performed to the ability i want it too.
But what you gotta remember is the Mac's are very very diffrent from the techie world of PC's. Macs are like one big plug and play. You dont get a mac for things like overclocking and changing out graphics cards as you go along etc.... when the time comes you want to step up performance, you buy a new Mac.
The idea behind a Mac is that anyone is meant to be able to take it home and use it without nowing the ins and outs of a delicate OS like Windows, Its designed for people like Designers, DJ's, Pod casters, People who care more about their music playlist and editing there vacation footage, Small time bands..etc..etc.. All the sort of people who really dont wanna put there time into knowing there OS and computer specs and not having to worry about performance or what the hell it means by Winsock errors or what Dll files there missing...
And the big one, Not worry about viruses.
Hope this helps in someways. I went Mac about a year ago and i would never go back. At times its fun to try and find a windows equivilent of software unless you wanna put an emulator or back ground copy of windows on your mac. Which in my mind defeats the purpose of having a mac.
Another issue to take to mind is if your a gamer. The world of gaming in mac is alot diffrent, alot of main stream games these days still dont support anything else but Windows. And even though thats slowly changing, you will still find yourself in a mac store shopping for mac games.