If you do just "save" on lion and above the old version will still be available through the "versions" feature. If you save as new/export it'll make an entirely new file, and you can subsequently delete the old one.
Another option is crop the image, then duplicate (via finder) the image. This will also make an entirely new image, which will let you erase the old one.
Either way, when you crop an image in preview it doesn't securely erase any of the old data - it just puts the new data right overtop. If your crop results in less block usage in the filesystem, any unused blocks may be marked as free, but they wouldn't be "erased".
One thing to keep in mind: If you do a major crop and then "securely erase" the image, it may or may not erase all of the old data as well (depends if the blocks and block usage are the same).
On a side note - if security is this much of a concern why not use full disk encryption? Then none of this would matter. With file vault 2 and a modern SSD there's virtually no performance penalty.