At the risk of sounding like a snob, computers do not slow down because
of wear, they slow down because of obsolescence. If you are saving
money to upgrade an 8 year old machine, you are far better off to save
for a little longer and buy a new machine. Even if a new machine is a
"bargain basement" model from your local box store, it is a quantum leap
over what can be done to an 8 year old machine. We are not talking
about that much of a difference in money being spent.

I would seriously install a XFCE environment into the Ubuntu on your old
machine and not upgrade it's hardware at all. Putting money into that
old of a machine is throwing good money after bad.