Yes Leopard can run 64 bit applications, and you have 64 bit processor.
That doesn't mean Safari is a 64 bit app though. Leopard can run both 64 and 32 bit process seamlessly.
Very few applications out there would see any benefit from being 64 bit (only applications that need to access massive amounts of data).
The only application that I know of that is included with Leopard that is 64 bit is "Chess.app".