AIUI it's basically that the plan was similar to the PS2, where you have a "smart" CPU (Emotion Engine / Cell) and a "dumb" rasteriser (Graphic Synthesiser on PS2), with the Cell SPEs taking the role of the vector units on the EE for things like transform + lighting. But the rasteriser chip project failed.

Then they tried the 2 Cells and software rendering approach, to try and keep it as an "in house" solution.

Finally they went to Nvidia and we got the final Cell + RSX solution.