The AMD 64 processors will execute 64-bit code 100% in 64-bit mode. From what I understand they do have instructions that allow them to process 32-bit instructions in compatibility mode. This allows them to used for current and future operating systems and software.
The Intel EMT64 operates in much the same with (mainly because some of the technology is licenced to Intel by AMD).
The differences from what I understand (and if somebody else understands it better please add) is that the AMD process will execute 32-bit instructions in a 48-bit compatibility mode, while the Intel architectural limitations will execute 32-bit instructions in 32-bit mode.
So, best answer I can give is yes the AMD 64 is a "true" 64-bit processor, but perhaps not a "pure" one?
AMD 64 4000+:w/ Coolermaster Hyper 48 HSF DFI Lan Party nF4 SLI-DR: BIOS 03/10P /nForce: 6.53 2x512MB OZC PC3200 Platinum Rev. 2 Lat: 2-2-2-5 2 x Geforce 6800 GT -SLI 2 X Seagate Baracuda 120GB (Raid 1) Silverstone Zeus ST65ZS 650W Silverstone TJ05B-T Enclosure
The requeriment for a Processor to qualify as a 64 Bits Processor is that its General Purpose Registers are 64 Bits width, and the K8s got that. K8s got 48 Bits Memory Adress, but it can be extended anytime to 64 Bits with no problems as far that I know. After you got enough RAM to make that 48 Bits Memory Adress aren't enough, it will be surely a long time. By the way, x86 Instructions and x86-64 Instructions are the same as far that I know, just that when you run in x86-64 Mode it makes that the eight 32 Bits General Purpose Registers are extended to 64 Bits plus it adds another eight 64 Bits General Purpose Registers.