Look before you buy if * is compatible:
- GPU
- CPU
- Motherboard audio chipset
- Motherboard internet chipset
- Mouse (no kidding, my G5 don't work on Mac)
It shouldn't be too hard, the hardest is probably the GPU.
I was able to make an Hackintosh on my old computer, AMD AM2 x2 4200+ 2.2Ghz, wierd Acer computer motherboard and my 4850 512m.
The GPU is fully supported since you can find it on real mac and the rest just took me some time to set correctly.
The reason why I don't use it, my G5 mouse is not compatible with real mac, and I love my mouse.
http://i40.tinypic.com/219dt2a.jpg
Look at insanelymac.com on the forum, and use google to know the compatibility of your piece.