You can already get the 64-bit Intel processors. Here's one source. There are others. In fact, the CPUs in those machines are Intel's second iteration of Itanium processors; there apparently was quite a problem with the first generation, but I don't remember now what it was. (I never used them.)

I don't know much about 64-bit Windows, but other OSes have been 64-bit for years. The only one I have any knowledge about in regards to how it deals with 64-bit and 32-bit code is Solaris. It certainly allows you to run 32-bit code and 64-bit code simultaneously without any sort of slowdown or emulation. In fact, the 64-bit code runs just a littel slower, as it takes more effort to deal with that big of an address space. As such, most applications are still 32-bit applications unless it really needs some feature provided by 64-bit-ness. Of course, the kernel and drivers are all 64-bit, so that you still get all the advantages of moving that much data around at the CPU level. Some of these statements are likely to be a little wrong, as I'm not a kernel guru. Mark might be able to correct me on some of this, since he knows more about kernels, if less about Solaris.