There's some confusion out there with regards to the 64-bit client, and while there's not an official blue post today about it, there is a fair amount of information from previous blue posts from which we can put together a pretty accurate picture.
If you don't know what a 64-bit client is or have no idea in general what I'm talking about, Mat McCurley wrote a good piece on what this is all about
earlier in the month.
Jurannok (Support Forums)
People with 64bit systems will be able to run the 32bit client perfectly fine.
The 64bit client is entirely optional. It's also still going to be in beta after it's released to the live realms with 4.3.2. We'll still be working on it further after the patch.
Jurannok (Support Fourms)
After the patch there will be more information about how to install it [the 64 bit client] and set it up.
So the 64-bit client wasn't contained in the patch you all just downloaded; it's still in beta. The client that's available to download from the PTR forums right now doesn't work with the live and patched servers -- it throws the "Unable to validate game version" error. We're unsure exactly if Blizzard will be releasing a beta
64-bit client to use on the live
realms today, but we've reached out to ask and will update if and when we hear back.