if you're cloning, you'd need the new drive (obviously), the application to do it (something like Acronis TrueImage), and some time. if your computer can accommodate both drives, then you don't need a separate enclosure - just stuff them both inside the PC and start the clone. if you're on a laptop then yeah you'd need an enclosure.
i think you'd need to make sure that both old and new hard drive have the same interface (both are IDE, or both are SATA), so that you can easily swap in the new drive once the clone is done.
we did stuff like this at work for a few old machines (migrated from HDD to SSD), and it's quite simple with the proper program (we used Ghost)