If you can't afford the $100-$150 or so it's going to take to purchase Leopard, you're surely not going to afford the $600 or more it would cost to purchase a system board. For the cost of a system board, you could buy a brand new mac mini.
Not to mention the fact that an Intel board is not going to fit into a G4 without some modding involved along with replacing probably just about everything else, including the CPU, hard drives, video card...
A G4 is not really worth spending a penny more than it takes to get an OS installed.
imho - if you can't find a copy of Leopard for less than $150, that G4 goes up for sale and let someone else sink money into it.