iMacs typically have a laptop sort of internal configuration which means that unless something has drasticly changed, no the video cards will not be able to be changed by the user, they'll be soldered onto the logic board.
i would imagine if it was an upgradable part other than BTO they would have mentioned it with the release.
Yeah I wouldve imagined that the gfx card would be better though... But in macs they are better than they would be because macs have faster processors and better hardware. I saw a Mac Pro at the Apple store with a 6600 GT in it, (dont ask why, I dont know why they would have one in an MP) and it was playing vide with great quality. Better than the x1600 with Avivo . Apple does it again.