I have Snow leopard working perfectly with r640.
I had the same kernal panic problems but then manged to install using PCEFI V9 and then installed from within SL to new hard drive using r640. The install went perfectly and I now run 10A432 in 64bit mode with only one issue which is sleep and i've not even looked into that because it's not something i'm overly bothered about.

What i'm saying is that I don't think r640 is the problem, allthough I don't pretend to be an expert.

I'm not at my home pc at the moment but will post the guide I used along with the files needed.

Only 4 kexts used by the way.

Cheers.

PS - My signature says 10.5.8 but that is on a seperate drive. I didn't use the EFI partion with Snow Leopard but will be moving files there when i get some spare time.