Out of curiousity - can't you run the 32 bit version? You can on Linux, and I'm quite certain Win64 ships with 32 bit dlls for exactly this reason. It wouldn't have any performance hits apart from big memory jobs.
Thanks. I know that SoftImage has a 64bit version, since Houdini runs on linux, I guess the 64 Versin could be possible, and 64bit runs more efficiently. Yes, I can run 32bit Houdini on 64bit Windows, in fact very well.