I agree with MrsBytch. If you are looking for best bang of you're buck, go with the AMD system. The DDR400 ram you have is going to be a problem when trying to match up with the newer processors. If you ditch the memory, you do have options to go with a Core 2 Duo system, but I don't think it will be as cheap as an X2 system.....
What parts do you have that you could reuse? PC3200 memory is not worth much. How much do you have?
Is the pc intended for gaming? If so, start with the most important part, the vga card.
Perhaps it would be better to design from scratch. For a low end system, you can't beat the price from some of the mass market vendors. They must get the OS for almost nothing. If you have parts you don't use, then sell them on e-bay for what they are worth and use the funds for what you want.