Until next year when AMD releases the same APU technology in a single dye, with granted 4 instead of 8 cores. It will also have HSA, which I believe the Xbox lacks.

Xbox (2001) was pretty much a PC.

The first Xbox also had unified memory. If it was that similar to a PC, you'd expect there to be several good Xbox emulators around.

No, you wouldn't. There aren't even PC emulators that include 3d graphics, only emulators that provide graphics card passthrough or provide software to install inside the emulator to pass high-level graphics commands to the host's OpenGL implementation. Nobody has a binary-compatible GeForce (or Radeon) emulator.

The unified memory enabled developers to practically start using the GPU for things other than rendering. This in turn places higher requirements on an emulator.

None of the emulators you see today existed before someone sat down and started writing them. Your point about PC emulators is irrelevant since if you control the guest OS, it's just much more practical to install a pass-through driver.