Think about purchasing drives that make full use of the quickest ports on your Mac. This might be FireWire (support is fading), USB 3 (most common fast port) or even Thundebolt (warp speed but expensive).
Another tip would be to leave you LR Catalog(s) on your internal HD. Just move the Image Library to the new HD. Fast access to the Catalog enhances performance. As an aside, when your new external HD is up and running, move the LR Image Lbrary from the internal drive to the external drive using LR. This way LR will keep track of the new locations automatically.
The only thing you need to be aware of is, when you back up your new external HD, your LR Catalog(s) will be backed up to a different place (whereever your internal HD back up resides). You could set up LR so it's Catalog back up goes to you new external HD. This is what I do. In this case you have 4 copies of your LR Catalogs: 1 on your internal HD, 2 on your internal HD back up, 3 on the external drive and 4 on the external HD back up (or Raid in your case). Given the values of a LR Catalog, having four copies is not a luxury.