You won't notice a performance increase with a 64 bit proc unless the software is written to take advantage of it. Microsoft was supposed to have the 64bit version of XP out in mass by Christmas but it may not happen. The thing that makes the Athlon64/FX and the AMD Opteron-server version better than AMD's current line is due to several improvements in the architecture of the chip.
One of the biggest is an onboard memory controller running at the processor speed and no longer have to communicate with the North Bridge. Things like Hyper Transport as well as the addition of SSE2 are some other benefits.
Now with a 64 bit OS and 64bit app the proc can chew through data in 64 bit chunks as opposed to the 32 bit.
Be aware while the G5 proc is 64 bit Apple still does not have an OS to support 64 bit yet. Most likely they won't do it themselves and leave it to those BSDers to figure it out and then add it to a future OS version.
I've heard rumors the Prescott with the 775 format might have 64bit compatibility that can be enabled in certain motherboards. http://www.neoseeker.com/news/articles/ ... ware/2526/
The Prescott will initially ship in a format to fit in today's P4 boards, socket 478, but will evolve into a newer 775 pin format without pins. The connectors will look more like tiny spheres.
I recommend going to www.maximumpc.com
and asking this question in their forum under hardware. There are many people that can give you more technical details on the above processors.