I've never attempted what you are asking here, but have done quite a few mods that Apple wouldn't approve of to my Macs over time. Hardware hacking on the Mac is quite a bit harder than on a PC, but isn't that challenge what hacking is all about?
Ram and IDE hard drives on the newer Macs are pretty much the same on the PC so just about any source would do. Well there is always exceptions I guess. Some RAM that is out of Apple spec won't work with Apple Motherboards with BIOSs that lock out that RAM so you may want to go with Apple vendors. Also some older Mac Boards have SCSI controlers rather than
For a video card you can buy Mac specific cards or there are some Nvidia and ATI cards that can be flashed to work with the Mac. Look at either www.macspeedzone.com or www.xlr8yourmac.com for more information.
You can't buy a new Mac motherboard on the open market so you might try a used motherboard from:
If you go 2 slashdot.org (Real Geek News) and click on the apple link on the left, they discuss a person who was able to build a G4, and they talk about where to get all parts needed, and assembly instructions. Very informative.