Either one of the hardware configurations exceeds the minimum requirements for a home server. The i5 would be more energy efficient and require less cooling (more quiet). If you sell your old hardware now, while it is still worth something, you could build
your i5 for a little money out of your pocket. I'm waiting til February and will update my WHS to the i3 Sandybridge .
I don't think WHS is a fully plug and play where you install and remove hardware, besides hard drives. I'm planning on a fresh install.