The only real 'noticable' performance increases will come when native x64 binaries are doing very processing intensive jobs. Encoding video is probably as good of an example as any. 64-bit can cut a 8-hour encoding process down by 20% or more.
Another prime example would be in server applications, such as Active Directory. Because x64 has much higher addressing space, Windows is able to cache entire directory in RAM (directory composed of say 300,000 or more users), and we all know how much faster RAM is than hard drives