Try the insanelymac.com forum or the osx86 forum.
It is called hackintosh, and there are some threads about it here, but you will get more hep in a forum that's dedicated to hackintoshing.
It is not illegal, but it is a violation of the EULA. If you are just doing it for yourself, and buy a legit copy of OSx, Apple isn't going to really do anything about it. If you start building computers and selling them with OSx, then you will have the Apple attorneys barking at your door.
It is not a trivial task. If you are experienced building PCs, and troubleshooting hardware issues and debugging software, then it can be done, but it's not necessarily the most stable way to go.