IMHO, the best way to do this is get the new drive, hook it up as a slave in the computer and then XXClone the old drive to the new slave... Be sure to enable the options so it will be a boot drive. Then, when you move the old one out, the new one acts exactly the same as the old drive. Or, you can use the old one until it dies and then hook up the new one to boot from.
XXClone