Early testing by Microsoft shows a reduction in IOPS of roughly 70% comparing Exchange 2003 to E12 on the same hardware.

You may have already heard Microsoft’s recent announcement that the next version of Exchange, currently dubbed E12, will ship as a 64-bit application only to run on x64 CPUs. This may have left you wondering what x64 is, how 64-bit technology is going to benefit Exchange, or what you should be looking for when purchasing hardware now, so when E12 ships you are ready.

Continued: http://www.msexchange.org/tutorials/Why-64-Bit-Good-E12.html