Apple has no reason to buy AMD, they can hire all the talent, and they already have PA SEMI to make whatever chips they want. I think the first move will be an iphone chipset with great Graphics. But I think a desktop style GPU would also be a possibility.
They have the capability to make any GPU they want, and now they have the people to design it. As long as they write drivers so the GPU would work in windows they could be on to a winner.
Apple loves to have standard parts through their product lines, making their own GPU would allow them to have maximum performance for a minimum price... after all most mid ranged graphics cards are weakened versions of the top model. The only reason a Nvidia 295 costs £400 is because, its what people are willing to pay for a top line product. Apple could make their own GPU's as fast as possible and have them as standard on both a mac pro and an imac. The same for the Macbook Pro and the Macbook. If apple does this then the Mac could become the PC Game weapon of choice, giving people great graphics bang for their buck.
Just imagine what people who say "macs are just a nice box" will think if the first GPU to play Crysis on full settings is only available from apple!