As your first step I would recommend this sequence after you have your new WD Black and VISTA.
1. Disconnect the old 250 GB drive - at least its SATA data cable, maybe also its power cable. Do this at the mobo port so that the SATA_0 port is free for the new drive.
2. Mount the new drive in the case and hook it up to the SATA_0 port. In reality this particular port is not necessary, it just conforms to common default expectations.
3. Close up and do all your installation of VISTA to the WD Black, now the only HDD in your box.
4. When all is well, open up the case and reconnect your old drive. Boot to the BIOS Setup and just be sure that the Boot Priority is set to use the new drive, and makes no attempt to boot from the old one.
You may still have to deal with how your new OS finds and uses your existing applications because they won't be on the new drive, unless you re-installed them all.