The same rules apply for keys as they did with Vista. The key works with either platform, as long as you use the same version. The key for Pro x86 will also work for Pro x64...but only one at a time.
On a side note, unless you have a very specific piece of software or an unique, older hardware device without x64 support, there's little to no reason to avoid x64. There's a reason why most new PCs ship with it. Don't cling to the past for the "comfort" factor. Do so only if you have a unique or specific reason for doing so.