The 64 bit client is now available for people running a 64bit OS. This gives wow more access to ram and a few other really nice potential performance boosts. hit up the link for download links as well as how to tell if your running a 64 bit OS.
the download is small and you just put it in your main wow dir, it doesn't overwrite anything and you just need to make sure at the main launcher options the load 32 bit client box is not checked and your good. the patched the servers over night to let the new client authenticate :)