Rather than buy an external _enclosure_ (that is, something you have to open to "get at" the drive inside), you might consider one of these:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=sata+usb+dock&x=0&y=0
(various items shown for comparison)
The one I got is this:
http://www.amazon.com/Syba-Connecla...?ie=UTF8&s=electronics&qid=1253062702&sr=1-22
($30 shipped and it works fine)
Then get the "bare" hard drive of your choice. I often go to newegg.com for such things, great prices and very fast shipping. Dealmac.com also keeps track of drive prices.
Also download CarbonCopyCloner -- absolutely free, and one of the best tools out there for cloning drives.
Then do this:
1. Connect the dock, put in the bare drive, boot up
2. Use Disk Utility to intialize the new drive
3. Use CCC to "clone" the contents of your internal to the new drive
4. IMPORTANT STEP: Restart, hold down the "option" key and KEEP HOLDING IT DOWN until the "startup manager" appears. You should now see the icon for the dock and the new drive in it. Choose to boot from this TO BE SURE that you have a "good cloned drive" BEFORE you do the swap.
5. Now, do the swap
6. When "closed up", try rebooting WITHOUT the dock attached. Do you get "a good boot"?
7. If everything "looks the same" to you as before the swap, good show!!
You can now use the old drive and the dock to be your backup drive. It doesn't really matter that it isn't the same size as the new drive -- at least until you've got more on the new drive than the old one can hold.
In that case, you can also use CCC to do "incremental, selective" backups, choosing NOT to back up some of the material on the source drive. Yes, a little "triage" is required here (selecting that which is/is not important to you), but it works.
And the nice thing about having the USB/SATA dock is that you can also use to mount _other_ drives if needed.