What was installed on your new Mac Pro?
The one I bought a few weeks back had Leopard installed on it and had a Snow Leopard disk in the box.
If this is the case then you can use the same Snow Leopard disk on your old machine and I wouldn't lose any sleep over it.
If your new Mac Pro came with Snow Leopard installed on it then the restore disk will not work with your old machine.
on a side note, I would run to another forum for questions, this place is full of a holes. I mean run and don't look back.