The Atheros 5454 or 5006 work as long as your PCI-E bus is supported ~Grumble crappy Fuji Grumble~ as these are the standard Mac cards.
One thing I do reccomend, if you're going to splash out on ANY component, make it a real cinema display. That way Core Image and Quartz Extreme JUST WORK.
What else? Always go intel, AMD isn't as well supported of course, and Modern intels are easily their rivals. Dual C2D Xeons are good, as are the most powerful Core 2 models.
Integrated Intel Audio can be an issue with certain codecs, although Taruga is improving the support almost daily. Just about EVERY Firewire and Bluetooth chip work out of the box, along with USB2. Some ethernet chipsets are a hassle, so just be careful when you pick your board. SCSI Is not the best supported component around, so avoid it and stick with Intel Sata. Modems rarely work, so if you want to be able to say you have a perfect mac, get one without. PCI-E over AGP, DDR 2 without question, and yes, as I said. Always get a real Apple display if you can. The older perspex ones should work fine, although I haven't had a chance to grab one yet - They're all expensive or heavily bruised.
If you want to use it in iLife you'll have to get an iSight webcam, but otherwise Macam will let you use a normal webcam. Graphics tablets by Wacom are all supported, USB by their own drivers, Serial by mewans of TabletMagic - Outside of these, they almost always work as a mouse, but the pressure sensitivity, silkscreened buttons etc are a who-knows.
Also, learning the different keys is a pain in the ass, so get an Apple Keyboard, if not the mouse (Mice are a free-for-all on USB - I'd just go with a USB Wacom personally.) They're cheap enough for you not to have to worry about it, and you can get the bluetooth model as well, so great for typing on your lap.
If you need a serial port, OS X comes with support for PCI Serrial cards. . I'm working mine slightly differently - I have a PCI-Cardbus adaptor, a Cardbus-CompactFlash adaptor, and a Compactflash to Serial adaptor which allows me to then use my serial connector on my TabletPC, my G4 Powermac, and my Zaurus PDA.
Finally, Always get a DVD burner, they're fun.