Cell is not THAT good in GPU. They went from the 2xCell PS3 in 2004/2005 to the Cell RSX in 2006. They realized that Cell doesn't surpass even a cheap GPU in some tasks. Its a waste for a CPU, with can process complex codes, to only perform simple tasks that only requires brutal force, like a GPU.
What I find funny is that all the experience that IBM gained by developing the Cell was used to make the Xenos(x360's processor). There are even rumours about some secrets of Cell being revealed for the development of Xenos, like while Sony IBM Toshiba were developing the Cell, some workers from IBM worked in both projects. Xenos have 3 PPUs almost identical of the Cell one. Both have 3.2GHz. I think that Microsoft wanted to make a console easy to make ports from. By releasing X360 early, MS pretty much assured that would have a very good support for their plataform. For the very loyal developers, there is nothing that money can't buy...
Actually, I think that if the PS3 didn't have a Blu-Ray, but instead a 8800GTX as GPU, they could have outsold Xbox360by a larger margin at today.
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