Rather than bigger physical and virtual address space, really useful for processes that need to allocate about > 4 GiB of memory (and can drive to slight decrease of performance since math on 64 bit addresses is slower), the switch to x86-64 also doubles the general purpose and mmx registers, and here comes the major performance increase. It was not the case of SPARC, where SUN suggested 32-bit for particular binaries since they could be faster than 64 bit counterparts (and no additional features apart larger address space) nor the Alpha of cource since they never deployed a 32 bit version.