as it has been stated, the ps3 doesn't need a dedicated gpu. the cell is more than capable enough to do all the graphic processing as well as the collision detection and core mechanic processing. the gpu was added to make it easier for developers to "migrate" to the new cpu architecture as programming for the cell is a lot different than programming for the "standard" x86 architecture chips that are in most computers. hell, even uncharted 2 only uses the rsx for physics calculations, all the graphic processing is done on/by the cell.
i am all for pushing the technology envelope and seeing how far we can go. however, bumping up the picture quality with a geforce gtx 999 with 4tb of ram (fictitious obviously) is not really pushing the envelope. all that does is make things look "prettier." the cell was ahead of it's time when it was released, and using the same processor in the ps4 would not push any envelopes. i for one would like to see real life physics applied to every single object in a game.
for instance, if you shoot a rocket launcher at a building i would like to see every single brick react based on the forces directly affecting them. so every time you shoot it with a rocket launcher the bricks would react in a slightly different way because your position would change, if ever so slightly, and that would cause the forces on the bricks to change as well. we are starting to see more real physics in games, however at this point they only apply to certain parts/aspects of the game.
red dead redemption is a game coming out that uses almost real bullet physics to make you opponents react based on where they are hit. unlike most other shooting games like mw2 where you can kill someone by shooting them in the toe. i want to see events and actions that are not scripted and not "pre-animated," rather i would like to see everyone have a different experience when they play the same game because all the game objects/elements would react based on how/where/when the player affected them.
i have also read in a few places where sony is working on a different kind of processor for the next game system. a processor that will be a hybrid of the cell we now know and "traditional" multicore processors. they want to lower the learning curve so it won't take developers as long to produce aaa masterpieces.
one more thing, the ps4 better support ps1, ps2, and ps3 discs or it won't be a definite buy for me. the b/c is one of the main features that attracted me to the ps3 and away from the 360.