Ok first of all dump the rumors about the i7 thing, TDP of that thing is way too high. You'd be looking at a energy efficient i5 core, which is more likely to be the update after the coming one. Second, it isn't even sure if there will be quad cores (neither the whole 2x dualcore shebang, B-S). Its probably gonna be a CTO option on the top of the line model, and even then you migth still be looking at a high end DUAL core (3,4 GHz or something).
About the grahics, I have no idea, but the iMac has always had a little advantage over the mobile line so I don't think they'll put in a 9600GT. More likely a 9800GT or GTX+ (the energy efficient one). And finally, as said already, 24" inch isn't going to be smaller or larger, until there is a 26" model with 2560x1600 res, Apple isn't going to change that.
However I am pretty shure about the graphics thing to be a nice update. If they'd chosen the 9800gs/gt they wouldn't have to make a new cooling system/ redesigned cooling block.
My 0.02