Two comments on this. First, I think that if Apple ships base models with 256MB, they will will be upgradeable to 512MB or more of RAM, so I don't really think that shipping with only 256MB is that big of a deal, particularly given how cheap RAM is these days.
As for the choice of graphics cards, my money is on one of two possibilities: ATI Radeon X600 or GeForce 6600. Yes, I realize that the former is somewhat more questionable since it's a PCI-Express part. I think the minimum that the new models will see is an ATI Radeon 9600 part, I just don't think it's realistic for them to base it on the GeForce 5200 at this point in time.
By the way, my biggest reason for suspecting this is that Doom III is going to be huge, for both the game itself and for future licensing of its engine, and it's almost certainly coming to the Mac. How about a guest appearance from John Carmack to personally introduce an early alpha/beta of Doom III running on the new iMac?