Bristol_Jonesey
I noticed immediately after switching to 64 bit throughout when setting up a new orchestral template, using EWQLSO Platinum.
Previously, running 32 bit, the maximum number of instances of the old Kompakt player i could load was 14, giving me a realistic number of audio + midi tracks to about 20
 
Now, my template consists of 135 tracks, including about 35 midi tracks dedcated to key switching.
My system doesn't even blink when editing/playing/mixing.

The system has to play a few games as the OS and applications have to live in the same address space when in 32-bit mode, where in 64-bit mode it's all linear addressing. Gravy.

There is a big difference in the speed of arithmetic when using 64-bit numbers.  A 32-bit chip/OS (or a 64-bit chip running in 32-bit mode) can do 64-bit arithmetic, but it is a lot slower.  I don't know how common it is for the various plug-ins to use 64-bit numbers.  I wouldn't be surprised if that were quite common actually, but I don't have any data to support that.

Most plugins are using floating point operations for the sample processing pipeline so integer processing speed becomes barely relevant.

http://www.meldaproduction.com/audiotutorials/32vs64.php
 
I like this description.