> Basic/low level mapping shoule be close, if not the same
> Both run in Big Endian, so much of the code may be able
> to just pass through the emulator

Actually, all scalar instructions (i.e. all but the SIMD ones) should be directly executable by a physical G4.

> The SIMD instructions would not be but so hard

So you had a look at Gekko's SIMD instructions?

> maybe the G4's altivec could pick up some of the work.

Yes, that's what I wrote. But I don't think the G4's AltiVec unit is able to directly execute Gekko's SIMD instructions as that would mean Gekko's SIMD is a subset of AltiVec, which I strongly doubt. That means the SIMD instructions would have to be emulated by translating them into AltiVec code (or scalar code which would be even slower of course).
Btw, if anybody has a reliable source (IBM, Nintendo or game developers) showing Gekko's SIMD as being a subset of AltiVec/VMX I'd be glad to be corrected :-)

> Looking at Dolphin's source would be a good start.

I didn't have a look at it but I suspect that there's a JIT in place that translates scalar PPC code directly into (scalar) x86 code and Gekko SIMD code directly into SSE code. If my suspicion is correct we wouldn't gain much from looking at this part of Dolphin's source code.