Anything in specific you want to hear? I've done in a couple times in different ways. Part swapping is ridiculously easy.I've got macs where the only Apple part is the motherboard and its supporting electronics. Most High end PC servers from the mid 90s share parts with macs (50 pin scsi, RAM on occasion) More recent systems (G3's and later) use almost the exact same hardware. I'm typing this message on a system where the only part Apple created is the motherboard and a personality card. Everything else was retail, eBay, or scavenged from a PC graveyard I have access to.

PSUs tend to use the same voltages, so at worst you need create an adapter harness, drives all use the same standards (ATA or SCSI), Firewire and USB are set international standards. Anymore even RAM is fairly compatible. You'll hit driver issues on expansion cards and the occasional device but even those aren't so bad anymore. Many video cards can be flashed to work with a Mac.

Cases create problems but some boards like the Beige G3 MLB comply with set standards (Mini-ATX) Even on those that don't some sheet plastic, zip ties and a set of needle files can make it extremely easy to mod a Mac into a PC case. If thats what you're looking for, try XLR8yourmac for some write-ups (both for converting ATX PSUs to mac boards and for putting mac boards into PC cases)