The GPUs of the GameCube and the Wii have nothing to do with Radeon GPUs. They were designed by ArtX before they got bought by ATI (now AMD).
> and I expect Dolphin to support WiiU soon
This is a common misconception. Dolphin will never support emulating the Wii U — it would make no sense to do so from a technical standpoint. The GameCube and the Wii are very similar, the Wii U is a completely different beast.
> Part of it might be due to Nintendo picking architectures that aren't too alien to emulate on a standard PC.
Uh... while the CPU side is fairly standard (PPC 750 with just a few extensions like locked cache, paired singles, gather pipe, etc.) everything else is very much custom on GC/Wii.
The GPU is an undocumented mixed floating point (on the vertex side) / fixed point (on the pixel side) architecture, with very few programmable elements (TEV/TCG) and a lot of configurable state.
The DSP is a programmable 16/32/40 bits chip that kind of ressembles Motorola DSP architectures but still looks completely custom when comparing actual ISAs. The ISA of the DSP was actually fully reverse engineered by emulator and homebrew developers, there is 0 technical documentation about it outside of these communities.
And there are more things like that which are far from being standard when you compare to e.g. an Xbox or an Xbox 360.
