You are going to see a big jump in performance from your old computer. Windows 7 also runs much better than Vista.
As for selling your old computer, they aren't worth much beings you can buy a new one for about $300. I take my old ones to a charity like Boys and Girls clubs, Slavation Army etc. They put them to very good use and it saves me the problem of disposing of them.
What do the color stripes mean on your tires?
Brian Cooley tells you why you might see various color lines on the wheels of your automobile.