Hi Richard,
Quick answer - lots :-).
I believe on Windows, with TP5, it goes something like this at the moment :
32 bit OS : 2 GB
32 bit OS /3GB switch : 3 GB
64 bit OS : 4 GB
A 64 bit version of TG2 will have access to more RAM. If your scene is not coming up against any memory limitations then you probably won't see much of a performance increase, but a 64 bit version will give access to far more RAM and that will allow TG2 to render larger images with more complex scenes. There may be a bit of a performance increase because on Windows using 64 bit means you have access to more fast registers on the CPU, but generally speaking 64 bit support shouldn't be looked on as a performance thing, rather it will let you do far more with TG2 in terms of scene complexity.
Regards,
Jo