I have received many emails and searches to this blog regarding AutoCAD and 64 bit. One of the biggest misconceptions is that 64 bit will increase the units precision beyond the current 16 decimals or 64 bit floating point which AutoCAD has supported since the 80's. I even wrote a blog article about the 64 bit accuracy misconception a few years ago.
The benefits of 64 bit hardware with a 64 bit Windows operating system is mainly for the increased addressable memory for very large files as the floating point of 64 bit CPUs has not changed for Intel or AMD. Performance is not really a comparison differentiation point unless of course you have been using HUGE files where it has been swapping to disk. Some performance gains may be realized simply because many 64 bit machines have a faster bus, memory, and I/O than most of the 32 bit machines that most have.
Currently you can run the 32 bit AutoCAD on 64 bit hardware with a 32 bit operating system but not the 64 bit operating systems. We at Autodesk are researching multi-core processors, Windows Vista compatibility, and 64 bit support for future releases of AutoCAD.
If we do have a 64 bit AutoCAD in the future it will be made available first for customers to beta test for those approved beta sites registered at http://myfeedback.autodesk.com.