I'm assuming you currently have 1 HDD attached to one header on the MB and a DVD drive attached to the other?
I would remove the DVD drive and attach the new HDD drive. Boot the machine, and then prep the new drive. Copy everything from the old drive to the new drive.
I would then remove the old HDD, put it in a safe place and put the DVD drive back.
Or
Assuming your data from the old drive isn't too much, copy everything from your HDD to a flash drive. It's a pretty good idea to do that anyway (in technical terms it's called a "back up"

).
Remove your old HDD and put the new one in. Boot from the OS4.1 disc and copy everything across.