My main problem with you is that you fail to look at the criteria the OP has stated.
The OP has stated that when the iMac is purchased, his wife will "want some of the latest games."
If she wants the "latest games" now why wouldn't she want the latest games three years from now? The woman is not playing Bejeweled! When a component cannot be upgraded, consideration has to be made for the future.
There is not now, nor will there be in the next 2 to 3 years any game that will only run on the nVidia card. Sorry, but you're going to have to accept this fact. The OP wanted to be sure this was not the case.
If getting the max FPS of every game and money was no issue whatsoever I would be advising him to buy the 8800 as well.
With the information from this thread and benchmark test information from Barefeats, I can safely say the 8880 is much faster for 3D gaming and greatly preferable in this particular situation. In three years the 8800 will still be twice as fast as the 2600 Pro; in three years the "latest games" will be even more demanding and since those are the type the wife plays, she wants the best GPU possible.
Will it still be 22% slower in Core Image/Core Video apps in three years?
What can you safely say about that? It's been a few months in the Mac Pro now.