Won't do you a lot of good, because the motherboards in iMacs are almost certainly a custom job, since Apple uses EFI instead of a BIOS. My best guess would be that they're made by Intel though.

The CPU should be right there on the main specs page. If you want the exact model number, you should be able to deduce it by whether or not it's a dual or quad core chip and then the clock speed. Like, the Q9300 is the only 2.5GHz Core 2 Quad chip there is last I checked. So if I know it's a quad core 2.5GHz CPU, I can be fairly sure it's a Q9300 CPU.

iMacs have no real internal expandability. You could swap out the hard drive and add RAM, but that's really about it. Everything else has to be handled by USB or Firewire devices. If you see one in a store you'll understand, since it's barely any thicker than your average LCD monitor.

Long story short, the iMac is an all-in-one solution. It offers little to no expandability, so what you see is what you get pretty much. If you want something resembling a PC with lots of internal expansion options, look at the Mac Pro line. It is a pretty big step up in price, but a lot of that is your paying for a high quality build. Those cases are the same from the G5 PowerMac days, and they are not some flimsy aluminum that will get all bent out of shape if you breathe too hard on it. You could drop that thing off a roof and probably barely dent it. You have dual quad core Xeon CPUs that are water cooled, ECC RAM which has built-in error checking and can be expanded to 32GB. Plenty of drive bays for adding more hard drives, etc. And one other nice thing about those cases, is they are incredibly easy to work on. Everything is laid out very nicely. Quite possibly even better than Apple's old "Quicksilver" line of Macs where the motherboard was mounted on a side plate that would just fold out when you opened the case. Made it oh so easy to get at the motherboard and have all the room you needed.

Anyway, the Mac Pro is your best bet if you want something resembling a PC. The iMac and Mac Mini both have very limited options for internal expansion. Think of them as laptops in a desktop configuration, because that is basically what they are. Sadly Apple really doesn't have any mid-range systems like the iMac, only with a more standard desktop config.

There is also always the "Hackintosh" route, but given the questionable legality of that route, you're on your own to research it and implement it if you so choose.