Windows patch 4.3.3 64-bit client download link
Mac patch 4.3.3 64-bit client download link
As a reminder, the 64-bit client is also considered a beta product, even though you can use it on the live realms. But with that said, most people experience only good things and no real problems with the 64-bit version of WoW. But if you do run into problems, be sure to let Blizzard know about them.
If you have no idea what any of this means, take a look at our brief explainer of the difference between 32- and 64-bit game clients.
The full Blizzard post after the break.