There will not be much, if any, speed differences for general use.
64-bit does not mean double the performance of a 32-bit system. It
allows the software to address more memory, and at a larger chunk at
a time.

With a large database or server, a 64-bit CPU and OS makes more sense,
and will notice a small increase in speed.

If you run CPU intensive programs, such as rendering video or working with
Photoshop, you will see a difference (assuming that the programs that are
written to 64 bit standards). Not many programs are now at that stage in
their development. The developers have been holding off, waiting to see if
the public demands such. Also, there are some programs that will just not
see much benefit by porting them to 64 bit.

Also, many/most companies will not want to have two versions of every
program they sell. The companies will likely hold off till Vista 64 bit has
proliferated the market.