Athlon 64 fixes a lot about the x86 ISA that was wrong in the first place. Sure, it isn't the nicest ISA in existence, but now it at least has a reasonable number of registers, etc.
Power4 is nice as well - lets talk about the IBM 970 however as this is a more realistic competitor. A very nice chip as well, it should compete and compare very well with Hammer at similar clock speeds. They both use next generation interfaces to the rest of the system (HyperTransport for Hammer, possibly HyperTransport for the IBM970, but it could be something similar instead), the Hammer has an onboard memory controller (or two) (dunno about the IBM970 here), they are both designed to scale very well (8 processors without special hardware for Hammer, 16 for IBM970), etc.
