Yep, what Al said -- in the same way that all 16-bit Windows 3.1 applications got re-written to be 32-bit applications under 32-bit Windows 9x in order to take advantage the then new 32-bit hardware.
The above is true for the 64-bit application.
The 64-bit engine in Sonar v5, (which works with either the 32 or 64-bit applications), allows double the accuracy and detail in the maths used behind the interface of Sonar. The end result for you is smoother, more detailed sounds -- particularly if using a lot of tracks/plugs.
Ciao,
B.