You can't really build Macs the way you build PCs (at least not anymore), since only Apple makes and sells the motherboards. Once you have a motherboard, though, you can install your own processors, RAM, drives and PCI/AGP cards, within the limitations of that particular board.
For example, helsinki, my warhorse Power Mac 7300, started life as a plain jane 180MHz PowerPC 604 (this computer was manufactured in 1997). I pulled the processor card and installed first a 400MHz PowerPC G3, then a 500MHz G3. I also upgraded the RAM from 64MB to 512MB, installed an ATI Rage 128 PCI accelerator (this was about three years ago), and installed a PCI card to add IDE, FireWire and USB (the Mac already had built-in SCSI and 10Mbit Ethernet). When I retired it as my workstation and turned it into a server, I pulled the Rage video card and replaced it with a 10/100 NIC, and installed NetBSD on it.
Similarly, my PowerBook 1400 started as a 1400cs/117 (a 117MHz PowerPC 603e). I installed a video-out option, a PCMCIA Ethernet card (with a driver I hacked myself, see
http://www.floodgap.com/retrotech/mac/enet3c589/ ), a PCMCIA modem, a G3/333 CPU upgrade, extra RAM, and a new active matrix screen. Once I get around to it, I'll put a G3/466 in it, and maybe a bigger IDE notebook drive.
The point is that while Mac motherboards come only from one source, you still have some hackability. There's just certain upgrades that aren't possible on some models (like the iMac and eMac really weren't designed to be oozing power).
I'll let you in on a secret, though -- I like being the "other 6 percent" (depending on who you talk to) of the market, not the other portion. We get much less crappy software in the Mac world. People program either for the Mac because they either have enough money to do the port right, and/or they just like to program for it and they do the job right out of pride. Virus authors aren't going to bother with a marginalized platform, and even if they did, OS X is arguably more secure intrinsically than Windows anyway. Apple does a good job on the synergy of software and system, something not possible if they were the mass market monster. I think I'd have some of the same complaints about Apple that I have about Microsoft today if they ruled the market instead of Gates.
After all that, you can have my Macs after you pry them from my cold, dead fingers. My entire server network is now 100% Mac (after the SPARC's hard disk finally died), almost evenly split between PowerPC and 68K. I'll be getting in a Sawtooth G4 with my tax refund (yes!) to do network backup services. My dual 1.25GHz G4 kicks most Pentium 4 butt, and runs continuously for months on end without a crash (it only loses uptime because *I* reboot it, not because it panicked). Yes, I like thinking different.
"you're a doctor.... and 27 years.... so...doctor + 27 years = HATORI SOHMA" - RoyalWing, when I was 27
"Al hail the forum editting Shooby! His vibes are law!" - Osaka-chan
I could still be champ, but I'd feel bad taking it away from one of the younger guys. - George Foreman