To the question regarding whether one put can a PC motherboard in G5 tower: the answer is yes.
I placed a gigabyte ga-945gcmx-s2 with 6700 intel into the G5 (it was one I took out of a tower I was updating). The G5 was one that I found at a Goodwill store. There was nothing inside except a fried motherboard. It has four 120mm fans which I added. You will have to rewire the switch but the computer now lights up and turns on. It also has a 500w power supply.
This is my first attempt at something like this. I am planning to upgrade the processor to i7 next month. By the way, I recommend that you do some planning before doing any cutting. That is something I wish I would have done.