This has been my experience, too, mostly. I ran both a 32-bit and a 64-bit Linux desktop on an Athlon64, both for several months, and I did not notice a differe

This has been my experience, too, mostly. I ran both a 32-bit and a 64-bit Linux desktop on an Athlon64, both for several months, and I did not notice a difference in performance. About the only real benefit is that the system can handle more RAM (which of course is a good thing). For some applications that are very, very memory-hungry, this can improve performance dramatically, because the program no longer needs to juggle all that data and can just keep it in RAM.