Joe: Your old hd placed in an external enclosure will likely not be accessible without your investment into some heavy software (read expensive), since it is a formatted drive, your new registry would not be able to 'open' it. What I would do is grab a cheap power supply and get the old box running. Then backup all of your files to either an external hd, dvd/cd or an online service, ie carbonite. Cheap ps cost about $30 around here.
You will then have complete set of files for your new build.
S