There's a lot of nonsense in here with regards to graphics cards, so I'll try to clear a few things up:
1) First of all, apple *will* go ATI, all the clues point to it and it makes the most sense because they currently offer the best cards when it comes to power-consumption/performance. NVIDIA is some 2 generations behind, this has been said/explained many times.
2) They will not use anything below ATI 5650. Again, this is obvious, and people who *know* graphics processors will only agree. It is all about power usage (watts). That alone is what apple's decision will be based on. The current 9600M GT uses 23 watts, so we can expect them to use a 5xxx card in the 15-25 watt range. This narrows it down to four cards: 5650/5730/5750/5830. These are the only 4 options apple has. I predict the 5650 because it alone offers excellent ("revolutionary") power usage: 4-8 watts lower than the current 9600 and almost 2x+ the graphics power.
3) They will, most likely, introduce auto-switchable graphics a la optimus, except it'll be an ATI version compatible with OSX.
Edit: someone mentioned the 5430 - that just shows the poster knows nothing about mobile GPUs. The card is almost a year old, weaker (or ~equal to) than the current 9600M GT and uses 7 (!) watts of power. At best, that's something the 13" could get, but even that's doubtable.