1. In 64bit, the CPU can fill a 64bit register in one go. You will only see a performance gain in area where complex mathematics are used (eg: video rendering, encoding/decoding video, encryption, archiving, mathematical modeling), because the algorithms are simple to perform mathematical operations on large values (values larger than 2^32 for example).
If you need a performance comparison; I used 32bit and 64bit 7-zip program decompress a 400MB+ cabinet file, which comes inside Acrobat Professional setup CD. In 32bit one, it took 2 minutes and 20 seconds. In 64bit one, it took 1 minute and 22 seconds, which is 70% faster.
Also, now, Windows for example, can address more than 4GB RAM without using PAE (Physical Address Extension) In fact, 32bit Windows XP for example, can't even show more than 3.5GB more of the times, because the address space also includes memory for device drivers and video RAM.
64bit OSes are more secure as well. This is not really because of the OS, but because a 64bit OS is required to unleash the hardware security features such as hardware based Data Execution Protection. Address Space Layout Randmonizer, a new feature in Vista is also available only in 64bit editions.