I think you would have a much harder time upgrading components in a Mac than you would in a Windows system. I also build my own machines and upgrade the parts. I couldn't imagine trying to do that with one of Jobs' machines (maybe you can and it would be just as easy, but I doubt it, I would be very uncomfortable trying to monkey around with a proprietary machine) - and there is the big MSFT/pc advantage. People feel comfortable playing around with them.
After I posted, I was sitting here wondering what I couldn't do if I had a big Apple tower. Do they have TV cards? I don't know, but as I type this, I'm watching ESPN2 NFL Films Superbowl 1984 (Raiders/Redskins) on my desktop. I bet the rest of my hardware (hard disks, mouse, keyboard, monitor) would hook up fine. But I don't know.
I get a lot of PC hardware dirt cheap on eBay. Apple doesn't seem to have anything like that kind of market depth.
The only thing proprietary on a Mac is a ROM or two, and the higher UI levels of the operating system. The CPU, memory, hard disks, optical drives, and USB/FIreWire connections are all the same as on a PC. I've upgraded all of these from time to time. Graphics cards are slightly different, but use the same AGP or PCI-X slots as on a PC.
-ccm
Hmm, I had just that exact experience working on a friend's Dell PC.