Re: Windows 7 Beta leaked
It is almost impossible for the x64 version to be slower than the x86 version. The tiny tiny overhead of thunking will be nothing compared to the advantage of having more GPRs. The only issues you can have with performance on x64 is bad driver optimizations but I think they have mostly gone away. And the advantage of using the additional memory (even as a cache) will be huge (even 700 MB as someone mentioned earlier).
This is not to say its not possible. I am running x64 and x86 W7 side by side (different specs though) and I don't feel any difference (subjectively).