Hmm, I don't think you need to have every component of the system 64bit-aware. Cubase SX as a standalone application will definitely perform better on a 64bit-CPU (name it Athlon64). 64bit drivers are not needed, neither is a 64bit Windows kernel. The Windows version may have some influence to the user interface rendering, but the audio performance isn't touched at all.
Regarding the performance of VST instruments and fx plugins, special versions are definitely needed (not for making them work with Cubase SX when running in 64bit mode, but for getting a remarkable performance boost).
I own SX3, but unfortunately no 64bit-capable machine ...
Hopefully next month this will change
I'll tell ya if it makes a difference.
I'm pretty sure you will find informations about this at the Steinberg forum http://forum.cubase.net