Remember when the iMac had a graphics option? The 7300 was standard, but you could upgrade to a 7600. I thought that was a great idea, but if Apple hasn't done it since I'd guess that people didn't take advantage of the option. Based on that speculation I think it's safe to say that they won't offer two cards again.
The 9600GT that was just released is a good piece of hardware, a solid jump. Unlike the 8600GT of today, which is close enough to the 7600. With any luck Apple will ditch ATI in the iMacs and go with the 9600m in the future.
On DDR3 with Montevina: Go check some benchmarks. DDR3 performance matches DDR2, because the latencies are much higher. Don't expect anything special because of DDR3.
I think WiMax is going to be part of the Centrino 2 package (Centrino is Intel's complete mobile networking solution, which Apple has never used).
The big thing about the Montevina board is that it's going to be much smaller than the current Santa Rosa board, which may allow Apple some "creative license" to make laptops thinner, or smaller if they like. iMacs won't become thinner because that would mean messing with the case, which they won't do so soon.